diff --git a/appengine/appengine.go b/appengine/appengine.go
index dcd363c..1f8706e 100644
--- a/appengine/appengine.go
+++ b/appengine/appengine.go
@@ -17,6 +17,7 @@
 package main
 
 import (
+	"fmt"
 	"github.com/barakmich/glog"
 	"os"
 	"time"
@@ -45,11 +46,11 @@ var (
 	timeout            = 30 * time.Second
 )
 
-func configFrom(file string) *config.Config {
+func configFrom(file string) (*config.Config, error) {
 	// Find the file...
 	if file != "" {
 		if _, err := os.Stat(file); os.IsNotExist(err) {
-			glog.Fatalln("Cannot find specified configuration file", file, ", aborting.")
+			return nil, fmt.Errorf("Cannot find specified configuration file", file)
 		}
 	} else if _, err := os.Stat("/cayley_appengine.cfg"); err == nil {
 		file = "/cayley_appengine.cfg"
@@ -59,7 +60,7 @@ func configFrom(file string) *config.Config {
 	}
 	cfg, err := config.Load(file)
 	if err != nil {
-		glog.Fatalln(err)
+		return nil, err
 	}
 
 	if cfg.DatabasePath == "" {
@@ -92,15 +93,19 @@ func configFrom(file string) *config.Config {
 
 	cfg.ReadOnly = cfg.ReadOnly || readOnly
 
-	return cfg
+	return cfg, nil
 }
 
 func init() {
 	glog.SetToStderr(true)
-	cfg := configFrom("cayley_appengine.cfg")
+	cfg, err := configFrom("cayley_appengine.cfg")
+	if err != nil {
+		glog.Fatalln("Error loading config:", err)
+	}
+
 	handle, err := db.Open(cfg)
 	if err != nil {
-		glog.Fatal(err)
+		glog.Fatalln("Error opening database:", err)
 	}
 	http.SetupRoutes(handle, cfg)
 }
diff --git a/graph/bolt/all_iterator.go b/graph/bolt/all_iterator.go
index 4cd017e..d8fae27 100644
--- a/graph/bolt/all_iterator.go
+++ b/graph/bolt/all_iterator.go
@@ -33,6 +33,7 @@ type AllIterator struct {
 	dir    quad.Direction
 	qs     *QuadStore
 	result *Token
+	err    error
 	buffer [][]byte
 	offset int
 	done   bool
@@ -121,6 +122,7 @@ func (it *AllIterator) Next() bool {
 		})
 		if err != nil {
 			glog.Error("Error nexting in database: ", err)
+			it.err = err
 			it.done = true
 			return false
 		}
@@ -134,6 +136,10 @@ func (it *AllIterator) Next() bool {
 	return true
 }
 
+func (it *AllIterator) Err() error {
+	return it.err
+}
+
 func (it *AllIterator) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
 }
@@ -168,10 +174,11 @@ func (it *AllIterator) Contains(v graph.Value) bool {
 	return true
 }
 
-func (it *AllIterator) Close() {
+func (it *AllIterator) Close() error {
 	it.result = nil
 	it.buffer = nil
 	it.done = true
+	return nil
 }
 
 func (it *AllIterator) Size() (int64, bool) {
@@ -204,3 +211,5 @@ func (it *AllIterator) Stats() graph.IteratorStats {
 		Size:         s,
 	}
 }
+
+var _ graph.Nexter = &AllIterator{}
diff --git a/graph/bolt/iterator.go b/graph/bolt/iterator.go
index efb11d6..d080391 100644
--- a/graph/bolt/iterator.go
+++ b/graph/bolt/iterator.go
@@ -50,6 +50,7 @@ type Iterator struct {
 	offset  int
 	done    bool
 	size    int64
+	err     error
 }
 
 func NewIterator(bucket []byte, d quad.Direction, value graph.Value, qs *QuadStore) *Iterator {
@@ -105,10 +106,11 @@ func (it *Iterator) Clone() graph.Iterator {
 	return out
 }
 
-func (it *Iterator) Close() {
+func (it *Iterator) Close() error {
 	it.result = nil
 	it.buffer = nil
 	it.done = true
+	return nil
 }
 
 func (it *Iterator) isLiveValue(val []byte) bool {
@@ -170,6 +172,7 @@ func (it *Iterator) Next() bool {
 		if err != nil {
 			if err != errNotExist {
 				glog.Errorf("Error nexting in database: %v", err)
+				it.err = err
 			}
 			it.done = true
 			return false
@@ -184,6 +187,10 @@ func (it *Iterator) Next() bool {
 	return true
 }
 
+func (it *Iterator) Err() error {
+	return it.err
+}
+
 func (it *Iterator) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
 }
@@ -316,3 +323,5 @@ func (it *Iterator) Stats() graph.IteratorStats {
 		Size:         s,
 	}
 }
+
+var _ graph.Nexter = &Iterator{}
diff --git a/graph/bolt/quadstore.go b/graph/bolt/quadstore.go
index 5b0e2e2..ecc9a81 100644
--- a/graph/bolt/quadstore.go
+++ b/graph/bolt/quadstore.go
@@ -92,7 +92,10 @@ func newQuadStore(path string, options graph.Options) (graph.QuadStore, error) {
 	}
 	qs.db = db
 	// BoolKey returns false on non-existence. IE, Sync by default.
-	qs.db.NoSync, _ = options.BoolKey("nosync")
+	qs.db.NoSync, _, err = options.BoolKey("nosync")
+	if err != nil {
+		return nil, err
+	}
 	err = qs.getMetadata()
 	if err != nil {
 		return nil, err
diff --git a/graph/gaedatastore/iterator.go b/graph/gaedatastore/iterator.go
index ffdc8fe..8f3ef18 100644
--- a/graph/gaedatastore/iterator.go
+++ b/graph/gaedatastore/iterator.go
@@ -41,6 +41,7 @@ type Iterator struct {
 	offset int
 	last   string
 	result graph.Value
+	err    error
 }
 
 var (
@@ -129,12 +130,13 @@ func (it *Iterator) Reset() {
 	it.result = nil
 }
 
-func (it *Iterator) Close() {
+func (it *Iterator) Close() error {
 	it.buffer = nil
 	it.offset = 0
 	it.done = true
 	it.last = ""
 	it.result = nil
+	return nil
 }
 
 func (it *Iterator) Tagger() *graph.Tagger {
@@ -267,7 +269,8 @@ func (it *Iterator) Next() bool {
 		}
 		if err != nil {
 			glog.Errorf("Error fetching next entry %v", err)
-			break
+			it.err = err
+			return false
 		}
 		if !skip {
 			it.buffer = append(it.buffer, k.StringID())
@@ -288,6 +291,10 @@ func (it *Iterator) Next() bool {
 	return true
 }
 
+func (it *Iterator) Err() error {
+	return it.err
+}
+
 func (it *Iterator) Size() (int64, bool) {
 	return it.size, true
 }
@@ -323,3 +330,5 @@ func (it *Iterator) Stats() graph.IteratorStats {
 		Size:         size,
 	}
 }
+
+var _ graph.Nexter = &Iterator{}
diff --git a/graph/gaedatastore/quadstore.go b/graph/gaedatastore/quadstore.go
index 10dfb39..8a7b12d 100644
--- a/graph/gaedatastore/quadstore.go
+++ b/graph/gaedatastore/quadstore.go
@@ -154,7 +154,7 @@ func getContext(opts graph.Options) (appengine.Context, error) {
 	req := opts["HTTPRequest"].(*http.Request)
 	if req == nil {
 		err := errors.New("HTTP Request needed")
-		glog.Fatalln(err)
+		glog.Errorln(err)
 		return nil, err
 	}
 	return appengine.NewContext(req), nil
diff --git a/graph/iterator.go b/graph/iterator.go
index 01a0358..281ea64 100644
--- a/graph/iterator.go
+++ b/graph/iterator.go
@@ -100,6 +100,9 @@ type Iterator interface {
 	// Contains returns whether the value is within the set held by the iterator.
 	Contains(Value) bool
 
+	// Err returns any error that was encountered by the Iterator.
+	Err() error
+
 	// Start iteration from the beginning
 	Reset()
 
@@ -136,7 +139,7 @@ type Iterator interface {
 	Describe() Description
 
 	// Close the iterator and do internal cleanup.
-	Close()
+	Close() error
 
 	// UID returns the unique identifier of the iterator.
 	UID() uint64
@@ -155,7 +158,9 @@ type Description struct {
 
 type Nexter interface {
 	// Next advances the iterator to the next value, which will then be available through
-	// the Result method. It returns false if no further advancement is possible.
+	// the Result method. It returns false if no further advancement is possible, or if an
+	// error was encountered during iteration.  Err should be consulted to distinguish
+	// between the two cases.
 	Next() bool
 
 	Iterator
diff --git a/graph/iterator/all_iterator.go b/graph/iterator/all_iterator.go
index d937ce4..26e6302 100644
--- a/graph/iterator/all_iterator.go
+++ b/graph/iterator/all_iterator.go
@@ -55,7 +55,9 @@ func (it *Int64) Reset() {
 	it.at = it.min
 }
 
-func (it *Int64) Close() {}
+func (it *Int64) Close() error {
+	return nil
+}
 
 func (it *Int64) Clone() graph.Iterator {
 	out := NewInt64(it.min, it.max)
@@ -103,6 +105,10 @@ func (it *Int64) Next() bool {
 	return graph.NextLogOut(it, val, true)
 }
 
+func (it *Int64) Err() error {
+	return nil
+}
+
 // DEPRECATED
 func (it *Int64) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
@@ -160,3 +166,5 @@ func (it *Int64) Stats() graph.IteratorStats {
 		Contains:     it.runstats.Contains,
 	}
 }
+
+var _ graph.Nexter = &Int64{}
diff --git a/graph/iterator/and_iterator.go b/graph/iterator/and_iterator.go
index 3764600..418a469 100644
--- a/graph/iterator/and_iterator.go
+++ b/graph/iterator/and_iterator.go
@@ -30,6 +30,7 @@ type And struct {
 	checkList         []graph.Iterator
 	result            graph.Value
 	runstats          graph.IteratorStats
+	err               error
 }
 
 // Creates a new And iterator.
@@ -153,9 +154,14 @@ func (it *And) Next() bool {
 			return graph.NextLogOut(it, curr, true)
 		}
 	}
+	it.err = it.primaryIt.Err()
 	return graph.NextLogOut(it, nil, false)
 }
 
+func (it *And) Err() error {
+	return it.err
+}
+
 func (it *And) Result() graph.Value {
 	return it.result
 }
@@ -182,9 +188,26 @@ func (it *And) checkContainsList(val graph.Value, lastResult graph.Value) bool {
 	for i, c := range it.checkList {
 		ok = c.Contains(val)
 		if !ok {
+			it.err = c.Err()
+			if it.err != nil {
+				return false
+			}
+
 			if lastResult != nil {
 				for j := 0; j < i; j++ {
+					// One of the iterators has determined that this value doesn't
+					// match. However, the iterators that came before in the list
+					// may have returned "ok" to Contains().  We need to set all
+					// the tags back to what the previous result was -- effectively
+					// seeking back exactly one -- so we check all the prior iterators
+					// with the (already verified) result and throw away the result,
+					// which will be 'true'
 					it.checkList[j].Contains(lastResult)
+
+					it.err = it.checkList[j].Err()
+					if it.err != nil {
+						return false
+					}
 				}
 			}
 			break
@@ -241,10 +264,19 @@ func (it *And) NextPath() bool {
 	if it.primaryIt.NextPath() {
 		return true
 	}
+	it.err = it.primaryIt.Err()
+	if it.err != nil {
+		return false
+	}
 	for _, sub := range it.internalIterators {
 		if sub.NextPath() {
 			return true
 		}
+
+		it.err = sub.Err()
+		if it.err != nil {
+			return false
+		}
 	}
 	return false
 }
@@ -254,14 +286,23 @@ func (it *And) cleanUp() {}
 
 // Close this iterator, and, by extension, close the subiterators.
 // Close should be idempotent, and it follows that if it's subiterators
-// follow this contract, the And follows the contract.
-func (it *And) Close() {
+// follow this contract, the And follows the contract.  It closes all
+// subiterators it can, but returns the first error it encounters.
+func (it *And) Close() error {
 	it.cleanUp()
-	it.primaryIt.Close()
+
+	err := it.primaryIt.Close()
 	for _, sub := range it.internalIterators {
-		sub.Close()
+		_err := sub.Close()
+		if _err != nil && err == nil {
+			err = _err
+		}
 	}
+
+	return err
 }
 
 // Register this as an "and" iterator.
 func (it *And) Type() graph.Type { return graph.And }
+
+var _ graph.Nexter = &And{}
diff --git a/graph/iterator/and_iterator_test.go b/graph/iterator/and_iterator_test.go
index 3f96519..1a9c131 100644
--- a/graph/iterator/and_iterator_test.go
+++ b/graph/iterator/and_iterator_test.go
@@ -15,6 +15,7 @@
 package iterator
 
 import (
+	"errors"
 	"testing"
 
 	"github.com/google/cayley/graph"
@@ -138,5 +139,20 @@ func TestAllIterators(t *testing.T) {
 	if and.Next() {
 		t.Error("Too many values")
 	}
-
+}
+
+func TestAndIteratorErr(t *testing.T) {
+	wantErr := errors.New("unique")
+	allErr := newTestIterator(false, wantErr)
+
+	and := NewAnd()
+	and.AddSubIterator(allErr)
+	and.AddSubIterator(NewInt64(1, 5))
+
+	if and.Next() != false {
+		t.Errorf("And iterator did not pass through initial 'false'")
+	}
+	if and.Err() != wantErr {
+		t.Errorf("And iterator did not pass through underlying Err")
+	}
 }
diff --git a/graph/iterator/fixed_iterator.go b/graph/iterator/fixed_iterator.go
index 4a8afb9..fe112bc 100644
--- a/graph/iterator/fixed_iterator.go
+++ b/graph/iterator/fixed_iterator.go
@@ -63,7 +63,9 @@ func (it *Fixed) Reset() {
 	it.lastIndex = 0
 }
 
-func (it *Fixed) Close() {}
+func (it *Fixed) Close() error {
+	return nil
+}
 
 func (it *Fixed) Tagger() *graph.Tagger {
 	return &it.tags
@@ -143,6 +145,10 @@ func (it *Fixed) Next() bool {
 	return graph.NextLogOut(it, out, true)
 }
 
+func (it *Fixed) Err() error {
+	return nil
+}
+
 // DEPRECATED
 func (it *Fixed) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
@@ -186,3 +192,5 @@ func (it *Fixed) Stats() graph.IteratorStats {
 		Size:         int64(len(it.values)),
 	}
 }
+
+var _ graph.Nexter = &Fixed{}
diff --git a/graph/iterator/hasa_iterator.go b/graph/iterator/hasa_iterator.go
index 3788b40..a799bdd 100644
--- a/graph/iterator/hasa_iterator.go
+++ b/graph/iterator/hasa_iterator.go
@@ -52,6 +52,7 @@ type HasA struct {
 	resultIt  graph.Iterator
 	result    graph.Value
 	runstats  graph.IteratorStats
+	err       error
 }
 
 // Construct a new HasA iterator, given the quad subiterator, and the quad
@@ -152,7 +153,11 @@ func (it *HasA) Contains(val graph.Value) bool {
 		it.resultIt.Close()
 	}
 	it.resultIt = it.qs.QuadIterator(it.dir, val)
-	return graph.ContainsLogOut(it, val, it.NextContains())
+	ok := it.NextContains()
+	if it.err != nil {
+		return false
+	}
+	return graph.ContainsLogOut(it, val, ok)
 }
 
 // NextContains() is shared code between Contains() and GetNextResult() -- calls next on the
@@ -170,6 +175,7 @@ func (it *HasA) NextContains() bool {
 			return true
 		}
 	}
+	it.err = it.resultIt.Err()
 	return false
 }
 
@@ -185,7 +191,15 @@ func (it *HasA) NextPath() bool {
 	if it.primaryIt.NextPath() {
 		return true
 	}
-	result := it.NextContains()
+	it.err = it.primaryIt.Err()
+	if it.err != nil {
+		return false
+	}
+
+	result := it.NextContains() // Sets it.err if there's an error
+	if it.err != nil {
+		return false
+	}
 	glog.V(4).Infoln("HASA", it.UID(), "NextPath Returns", result, "")
 	return result
 }
@@ -202,6 +216,7 @@ func (it *HasA) Next() bool {
 	it.resultIt = &Null{}
 
 	if !graph.Next(it.primaryIt) {
+		it.err = it.primaryIt.Err()
 		return graph.NextLogOut(it, 0, false)
 	}
 	tID := it.primaryIt.Result()
@@ -210,6 +225,10 @@ func (it *HasA) Next() bool {
 	return graph.NextLogOut(it, val, true)
 }
 
+func (it *HasA) Err() error {
+	return it.err
+}
+
 func (it *HasA) Result() graph.Value {
 	return it.result
 }
@@ -238,12 +257,19 @@ func (it *HasA) Stats() graph.IteratorStats {
 	}
 }
 
-// Close the subiterator, the result iterator (if any) and the HasA.
-func (it *HasA) Close() {
+// Close the subiterator, the result iterator (if any) and the HasA. It closes
+// all subiterators it can, but returns the first error it encounters.
+func (it *HasA) Close() error {
+	err := it.primaryIt.Close()
+
 	if it.resultIt != nil {
-		it.resultIt.Close()
+		_err := it.resultIt.Close()
+		if err == nil {
+			err = _err
+		}
 	}
-	it.primaryIt.Close()
+
+	return err
 }
 
 // Register this iterator as a HasA.
@@ -252,3 +278,5 @@ func (it *HasA) Type() graph.Type { return graph.HasA }
 func (it *HasA) Size() (int64, bool) {
 	return it.Stats().Size, false
 }
+
+var _ graph.Nexter = &HasA{}
diff --git a/graph/iterator/hasa_iterator_test.go b/graph/iterator/hasa_iterator_test.go
new file mode 100644
index 0000000..d7498cf
--- /dev/null
+++ b/graph/iterator/hasa_iterator_test.go
@@ -0,0 +1,37 @@
+// Copyright 2014 The Cayley Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iterator
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/google/cayley/quad"
+)
+
+func TestHasAIteratorErr(t *testing.T) {
+	wantErr := errors.New("unique")
+	errIt := newTestIterator(false, wantErr)
+
+	// TODO(andrew-d): pass a non-nil quadstore
+	hasa := NewHasA(nil, errIt, quad.Subject)
+
+	if hasa.Next() != false {
+		t.Errorf("HasA iterator did not pass through initial 'false'")
+	}
+	if hasa.Err() != wantErr {
+		t.Errorf("HasA iterator did not pass through underlying Err")
+	}
+}
diff --git a/graph/iterator/iterator.go b/graph/iterator/iterator.go
index 954ddba..371025c 100644
--- a/graph/iterator/iterator.go
+++ b/graph/iterator/iterator.go
@@ -88,6 +88,10 @@ func (it *Null) Next() bool {
 	return false
 }
 
+func (it *Null) Err() error {
+	return nil
+}
+
 func (it *Null) Result() graph.Value {
 	return nil
 }
@@ -110,9 +114,13 @@ func (it *Null) Size() (int64, bool) {
 
 func (it *Null) Reset() {}
 
-func (it *Null) Close() {}
+func (it *Null) Close() error {
+	return nil
+}
 
 // A null iterator costs nothing. Use it!
 func (it *Null) Stats() graph.IteratorStats {
 	return graph.IteratorStats{}
 }
+
+var _ graph.Nexter = &Null{}
diff --git a/graph/iterator/iterator_test.go b/graph/iterator/iterator_test.go
new file mode 100644
index 0000000..d20635c
--- /dev/null
+++ b/graph/iterator/iterator_test.go
@@ -0,0 +1,29 @@
+package iterator
+
+import (
+	"github.com/google/cayley/graph"
+)
+
+// A testing iterator that returns the given values for Next() and Err().
+type testIterator struct {
+	*Fixed
+
+	NextVal bool
+	ErrVal  error
+}
+
+func newTestIterator(next bool, err error) graph.Iterator {
+	return &testIterator{
+		Fixed:   NewFixed(Identity),
+		NextVal: next,
+		ErrVal:  err,
+	}
+}
+
+func (it *testIterator) Next() bool {
+	return it.NextVal
+}
+
+func (it *testIterator) Err() error {
+	return it.ErrVal
+}
diff --git a/graph/iterator/linksto_iterator.go b/graph/iterator/linksto_iterator.go
index 99a8df5..930e929 100644
--- a/graph/iterator/linksto_iterator.go
+++ b/graph/iterator/linksto_iterator.go
@@ -46,6 +46,7 @@ type LinksTo struct {
 	nextIt    graph.Iterator
 	result    graph.Value
 	runstats  graph.IteratorStats
+	err       error
 }
 
 // Construct a new LinksTo iterator around a direction and a subiterator of
@@ -125,6 +126,7 @@ func (it *LinksTo) Contains(val graph.Value) bool {
 		it.result = val
 		return graph.ContainsLogOut(it, val, true)
 	}
+	it.err = it.primaryIt.Err()
 	return graph.ContainsLogOut(it, val, false)
 }
 
@@ -164,8 +166,17 @@ func (it *LinksTo) Next() bool {
 		return graph.NextLogOut(it, it.nextIt, true)
 	}
 
+	// If there's an error in the 'next' iterator, we save it and we're done.
+	it.err = it.nextIt.Err()
+	if it.err != nil {
+		return false
+	}
+
 	// Subiterator is empty, get another one
 	if !graph.Next(it.primaryIt) {
+		// Possibly save error
+		it.err = it.primaryIt.Err()
+
 		// We're out of nodes in our subiterator, so we're done as well.
 		return graph.NextLogOut(it, 0, false)
 	}
@@ -176,19 +187,34 @@ func (it *LinksTo) Next() bool {
 	return it.Next()
 }
 
+func (it *LinksTo) Err() error {
+	return it.err
+}
+
 func (it *LinksTo) Result() graph.Value {
 	return it.result
 }
 
-// Close our subiterators.
-func (it *LinksTo) Close() {
-	it.nextIt.Close()
-	it.primaryIt.Close()
+// Close closes the iterator.  It closes all subiterators it can, but
+// returns the first error it encounters.
+func (it *LinksTo) Close() error {
+	err := it.nextIt.Close()
+
+	_err := it.primaryIt.Close()
+	if _err != nil && err == nil {
+		err = _err
+	}
+
+	return err
 }
 
 // We won't ever have a new result, but our subiterators might.
 func (it *LinksTo) NextPath() bool {
-	return it.primaryIt.NextPath()
+	ok := it.primaryIt.NextPath()
+	if !ok {
+		it.err = it.primaryIt.Err()
+	}
+	return ok
 }
 
 // Register the LinksTo.
@@ -214,3 +240,5 @@ func (it *LinksTo) Stats() graph.IteratorStats {
 func (it *LinksTo) Size() (int64, bool) {
 	return it.Stats().Size, false
 }
+
+var _ graph.Nexter = &LinksTo{}
diff --git a/graph/iterator/materialize_iterator.go b/graph/iterator/materialize_iterator.go
index 00b2102..f6421d8 100644
--- a/graph/iterator/materialize_iterator.go
+++ b/graph/iterator/materialize_iterator.go
@@ -49,6 +49,7 @@ type Materialize struct {
 	hasRun      bool
 	aborted     bool
 	runstats    graph.IteratorStats
+	err         error
 }
 
 func NewMaterialize(sub graph.Iterator) *Materialize {
@@ -69,11 +70,11 @@ func (it *Materialize) Reset() {
 	it.index = -1
 }
 
-func (it *Materialize) Close() {
-	it.subIt.Close()
+func (it *Materialize) Close() error {
 	it.containsMap = nil
 	it.values = nil
 	it.hasRun = false
+	return it.subIt.Close()
 }
 
 func (it *Materialize) Tagger() *graph.Tagger {
@@ -108,6 +109,7 @@ func (it *Materialize) Clone() graph.Iterator {
 	if it.hasRun {
 		out.hasRun = true
 		out.aborted = it.aborted
+		out.err = it.err
 		out.values = it.values
 		out.containsMap = it.containsMap
 		out.actualSize = it.actualSize
@@ -199,8 +201,13 @@ func (it *Materialize) Next() bool {
 	if !it.hasRun {
 		it.materializeSet()
 	}
+	if it.err != nil {
+		return false
+	}
 	if it.aborted {
-		return graph.Next(it.subIt)
+		n := graph.Next(it.subIt)
+		it.err = it.subIt.Err()
+		return n
 	}
 
 	it.index++
@@ -211,12 +218,19 @@ func (it *Materialize) Next() bool {
 	return graph.NextLogOut(it, it.Result(), true)
 }
 
+func (it *Materialize) Err() error {
+	return it.err
+}
+
 func (it *Materialize) Contains(v graph.Value) bool {
 	graph.ContainsLogIn(it, v)
 	it.runstats.Contains += 1
 	if !it.hasRun {
 		it.materializeSet()
 	}
+	if it.err != nil {
+		return false
+	}
 	if it.aborted {
 		return it.subIt.Contains(v)
 	}
@@ -236,6 +250,9 @@ func (it *Materialize) NextPath() bool {
 	if !it.hasRun {
 		it.materializeSet()
 	}
+	if it.err != nil {
+		return false
+	}
 	if it.aborted {
 		return it.subIt.NextPath()
 	}
@@ -283,7 +300,8 @@ func (it *Materialize) materializeSet() {
 			it.actualSize += 1
 		}
 	}
-	if it.aborted {
+	it.err = it.subIt.Err()
+	if it.err == nil && it.aborted {
 		if glog.V(2) {
 			glog.V(2).Infoln("Aborting subiterator")
 		}
@@ -293,3 +311,5 @@ func (it *Materialize) materializeSet() {
 	}
 	it.hasRun = true
 }
+
+var _ graph.Nexter = &Materialize{}
diff --git a/graph/iterator/materialize_iterator_test.go b/graph/iterator/materialize_iterator_test.go
new file mode 100644
index 0000000..8188584
--- /dev/null
+++ b/graph/iterator/materialize_iterator_test.go
@@ -0,0 +1,70 @@
+// Copyright 2014 The Cayley Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iterator
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestMaterializeIteratorError(t *testing.T) {
+	wantErr := errors.New("unique")
+	errIt := newTestIterator(false, wantErr)
+
+	// This tests that we properly return 0 results and the error when the
+	// underlying iterator returns an error.
+	mIt := NewMaterialize(errIt)
+
+	if mIt.Next() != false {
+		t.Errorf("Materialize iterator did not pass through underlying 'false'")
+	}
+	if mIt.Err() != wantErr {
+		t.Errorf("Materialize iterator did not pass through underlying Err")
+	}
+}
+
+func TestMaterializeIteratorErrorAbort(t *testing.T) {
+	wantErr := errors.New("unique")
+	errIt := newTestIterator(false, wantErr)
+
+	// This tests that we properly return 0 results and the error when the
+	// underlying iterator is larger than our 'abort at' value, and then
+	// returns an error.
+	or := NewOr()
+	or.AddSubIterator(NewInt64(1, int64(abortMaterializeAt+1)))
+	or.AddSubIterator(errIt)
+
+	mIt := NewMaterialize(or)
+
+	// We should get all the underlying values...
+	for i := 0; i < abortMaterializeAt+1; i++ {
+		if !mIt.Next() {
+			t.Errorf("Materialize iterator returned spurious 'false' on iteration %d", i)
+			return
+		}
+		if mIt.Err() != nil {
+			t.Errorf("Materialize iterator returned non-nil Err on iteration %d", i)
+			return
+		}
+	}
+
+	// ... and then the error value.
+	if mIt.Next() != false {
+		t.Errorf("Materialize iterator did not pass through underlying 'false'")
+	}
+	if mIt.Err() != wantErr {
+		t.Errorf("Materialize iterator did not pass through underlying Err")
+	}
+}
diff --git a/graph/iterator/not_iterator.go b/graph/iterator/not_iterator.go
index db394f8..98232d6 100644
--- a/graph/iterator/not_iterator.go
+++ b/graph/iterator/not_iterator.go
@@ -13,6 +13,7 @@ type Not struct {
 	allIt     graph.Iterator
 	result    graph.Value
 	runstats  graph.IteratorStats
+	err       error
 }
 
 func NewNot(primaryIt, allIt graph.Iterator) *Not {
@@ -87,9 +88,14 @@ func (it *Not) Next() bool {
 			return graph.NextLogOut(it, curr, true)
 		}
 	}
+	it.err = it.allIt.Err()
 	return graph.NextLogOut(it, nil, false)
 }
 
+func (it *Not) Err() error {
+	return it.err
+}
+
 func (it *Not) Result() graph.Value {
 	return it.result
 }
@@ -105,6 +111,12 @@ func (it *Not) Contains(val graph.Value) bool {
 		return graph.ContainsLogOut(it, val, false)
 	}
 
+	it.err = it.primaryIt.Err()
+	if it.err != nil {
+		// Explicitly return 'false', since an error occurred.
+		return false
+	}
+
 	it.result = val
 	return graph.ContainsLogOut(it, val, true)
 }
@@ -115,9 +127,17 @@ func (it *Not) NextPath() bool {
 	return false
 }
 
-func (it *Not) Close() {
-	it.primaryIt.Close()
-	it.allIt.Close()
+// Close closes the primary and all iterators.  It closes all subiterators
+// it can, but returns the first error it encounters.
+func (it *Not) Close() error {
+	err := it.primaryIt.Close()
+
+	_err := it.allIt.Close()
+	if _err != nil && err == nil {
+		err = _err
+	}
+
+	return err
 }
 
 func (it *Not) Type() graph.Type { return graph.Not }
@@ -161,3 +181,5 @@ func (it *Not) Describe() graph.Description {
 		Iterators: subIts,
 	}
 }
+
+var _ graph.Nexter = &Not{}
diff --git a/graph/iterator/not_iterator_test.go b/graph/iterator/not_iterator_test.go
index ac63b7c..d701513 100644
--- a/graph/iterator/not_iterator_test.go
+++ b/graph/iterator/not_iterator_test.go
@@ -1,6 +1,7 @@
 package iterator
 
 import (
+	"errors"
 	"reflect"
 	"testing"
 )
@@ -42,3 +43,19 @@ func TestNotIteratorBasics(t *testing.T) {
 		}
 	}
 }
+
+func TestNotIteratorErr(t *testing.T) {
+	wantErr := errors.New("unique")
+	allIt := newTestIterator(false, wantErr)
+
+	toComplementIt := NewFixed(Identity)
+
+	not := NewNot(toComplementIt, allIt)
+
+	if not.Next() != false {
+		t.Errorf("Not iterator did not pass through initial 'false'")
+	}
+	if not.Err() != wantErr {
+		t.Errorf("Not iterator did not pass through underlying Err")
+	}
+}
diff --git a/graph/iterator/optional_iterator.go b/graph/iterator/optional_iterator.go
index bc65acc..dbf654c 100644
--- a/graph/iterator/optional_iterator.go
+++ b/graph/iterator/optional_iterator.go
@@ -38,6 +38,7 @@ type Optional struct {
 	subIt     graph.Iterator
 	lastCheck bool
 	result    graph.Value
+	err       error
 }
 
 // Creates a new optional iterator.
@@ -57,8 +58,8 @@ func (it *Optional) Reset() {
 	it.lastCheck = false
 }
 
-func (it *Optional) Close() {
-	it.subIt.Close()
+func (it *Optional) Close() error {
+	return it.subIt.Close()
 }
 
 func (it *Optional) Tagger() *graph.Tagger {
@@ -76,6 +77,10 @@ func (it *Optional) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
 }
 
+func (it *Optional) Err() error {
+	return it.err
+}
+
 func (it *Optional) Result() graph.Value {
 	return it.result
 }
@@ -85,7 +90,11 @@ func (it *Optional) Result() graph.Value {
 // optional subbranch.
 func (it *Optional) NextPath() bool {
 	if it.lastCheck {
-		return it.subIt.NextPath()
+		ok := it.subIt.NextPath()
+		if !ok {
+			it.err = it.subIt.Err()
+		}
+		return ok
 	}
 	return false
 }
@@ -101,6 +110,7 @@ func (it *Optional) SubIterators() []graph.Iterator {
 func (it *Optional) Contains(val graph.Value) bool {
 	checked := it.subIt.Contains(val)
 	it.lastCheck = checked
+	it.err = it.subIt.Err()
 	it.result = val
 	return true
 }
@@ -152,3 +162,5 @@ func (it *Optional) Stats() graph.IteratorStats {
 func (it *Optional) Size() (int64, bool) {
 	return 0, true
 }
+
+var _ graph.Iterator = &Optional{}
diff --git a/graph/iterator/or_iterator.go b/graph/iterator/or_iterator.go
index 6c58238..25524e6 100644
--- a/graph/iterator/or_iterator.go
+++ b/graph/iterator/or_iterator.go
@@ -33,6 +33,7 @@ type Or struct {
 	itCount           int
 	currentIterator   int
 	result            graph.Value
+	err               error
 }
 
 func NewOr() *Or {
@@ -147,6 +148,11 @@ func (it *Or) Next() bool {
 			return graph.NextLogOut(it, it.result, true)
 		}
 
+		it.err = curIt.Err()
+		if it.err != nil {
+			return graph.NextLogOut(it, nil, false)
+		}
+
 		if it.isShortCircuiting && !first {
 			break
 		}
@@ -159,12 +165,16 @@ func (it *Or) Next() bool {
 	return graph.NextLogOut(it, nil, false)
 }
 
+func (it *Or) Err() error {
+	return it.err
+}
+
 func (it *Or) Result() graph.Value {
 	return it.result
 }
 
 // Checks a value against the iterators, in order.
-func (it *Or) subItsContain(val graph.Value) bool {
+func (it *Or) subItsContain(val graph.Value) (bool, error) {
 	var subIsGood = false
 	for i, sub := range it.internalIterators {
 		subIsGood = sub.Contains(val)
@@ -172,15 +182,23 @@ func (it *Or) subItsContain(val graph.Value) bool {
 			it.currentIterator = i
 			break
 		}
+
+		err := sub.Err()
+		if err != nil {
+			return false, err
+		}
 	}
-	return subIsGood
+	return subIsGood, nil
 }
 
 // Check a value against the entire graph.iterator, in order.
 func (it *Or) Contains(val graph.Value) bool {
 	graph.ContainsLogIn(it, val)
-	anyGood := it.subItsContain(val)
-	if !anyGood {
+	anyGood, err := it.subItsContain(val)
+	if err != nil {
+		it.err = err
+		return false
+	} else if !anyGood {
 		return graph.ContainsLogOut(it, val, false)
 	}
 	it.result = val
@@ -221,7 +239,12 @@ func (it *Or) Size() (int64, bool) {
 // shortcircuiting, only allow new results from the currently checked graph.iterator
 func (it *Or) NextPath() bool {
 	if it.currentIterator != -1 {
-		return it.internalIterators[it.currentIterator].NextPath()
+		currIt := it.internalIterators[it.currentIterator]
+		ok := currIt.NextPath()
+		if !ok {
+			it.err = currIt.Err()
+		}
+		return ok
 	}
 	return false
 }
@@ -231,12 +254,20 @@ func (it *Or) cleanUp() {}
 
 // Close this graph.iterator, and, by extension, close the subiterators.
 // Close should be idempotent, and it follows that if it's subiterators
-// follow this contract, the And follows the contract.
-func (it *Or) Close() {
+// follow this contract, the Or follows the contract.  It closes all
+// subiterators it can, but returns the first error it encounters.
+func (it *Or) Close() error {
 	it.cleanUp()
+
+	var err error
 	for _, sub := range it.internalIterators {
-		sub.Close()
+		_err := sub.Close()
+		if _err != nil && err == nil {
+			err = _err
+		}
 	}
+
+	return err
 }
 
 func (it *Or) Optimize() (graph.Iterator, bool) {
@@ -289,3 +320,5 @@ func (it *Or) Stats() graph.IteratorStats {
 
 // Register this as an "or" graph.iterator.
 func (it *Or) Type() graph.Type { return graph.Or }
+
+var _ graph.Nexter = &Or{}
diff --git a/graph/iterator/or_iterator_test.go b/graph/iterator/or_iterator_test.go
index 27cdc7d..7c4f2ef 100644
--- a/graph/iterator/or_iterator_test.go
+++ b/graph/iterator/or_iterator_test.go
@@ -15,6 +15,7 @@
 package iterator
 
 import (
+	"errors"
 	"reflect"
 	"testing"
 
@@ -148,3 +149,46 @@ func TestShortCircuitingOrBasics(t *testing.T) {
 		t.Errorf("Failed to iterate optimized Or correctly, got:%v expect:%v", got, expect)
 	}
 }
+
+func TestOrIteratorErr(t *testing.T) {
+	wantErr := errors.New("unique")
+	orErr := newTestIterator(false, wantErr)
+
+	fix1 := NewFixed(Identity)
+	fix1.Add(1)
+
+	or := NewOr()
+	or.AddSubIterator(fix1)
+	or.AddSubIterator(orErr)
+	or.AddSubIterator(NewInt64(1, 5))
+
+	if !or.Next() {
+		t.Errorf("Failed to iterate Or correctly")
+	}
+	if got := or.Result(); got != 1 {
+		t.Errorf("Failed to iterate Or correctly, got:%v expect:1", got)
+	}
+
+	if or.Next() != false {
+		t.Errorf("Or iterator did not pass through underlying 'false'")
+	}
+	if or.Err() != wantErr {
+		t.Errorf("Or iterator did not pass through underlying Err")
+	}
+}
+
+func TestShortCircuitOrIteratorErr(t *testing.T) {
+	wantErr := errors.New("unique")
+	orErr := newTestIterator(false, wantErr)
+
+	or := NewOr()
+	or.AddSubIterator(orErr)
+	or.AddSubIterator(NewInt64(1, 5))
+
+	if or.Next() != false {
+		t.Errorf("Or iterator did not pass through underlying 'false'")
+	}
+	if or.Err() != wantErr {
+		t.Errorf("Or iterator did not pass through underlying Err")
+	}
+}
diff --git a/graph/iterator/value_comparison_iterator.go b/graph/iterator/value_comparison_iterator.go
index 627b853..0351d60 100644
--- a/graph/iterator/value_comparison_iterator.go
+++ b/graph/iterator/value_comparison_iterator.go
@@ -27,7 +27,6 @@ package iterator
 // In MQL terms, this is the [{"age>=": 21}] concept.
 
 import (
-	"log"
 	"strconv"
 
 	"github.com/google/cayley/graph"
@@ -51,6 +50,7 @@ type Comparison struct {
 	val    interface{}
 	qs     graph.QuadStore
 	result graph.Value
+	err    error
 }
 
 func NewComparison(sub graph.Iterator, op Operator, val interface{}, qs graph.QuadStore) *Comparison {
@@ -91,8 +91,8 @@ func (it *Comparison) doComparison(val graph.Value) bool {
 	}
 }
 
-func (it *Comparison) Close() {
-	it.subIt.Close()
+func (it *Comparison) Close() error {
+	return it.subIt.Close()
 }
 
 func RunIntOp(a int64, op Operator, b int64) bool {
@@ -106,8 +106,7 @@ func RunIntOp(a int64, op Operator, b int64) bool {
 	case compareGTE:
 		return a >= b
 	default:
-		log.Fatal("Unknown operator type")
-		return false
+		panic("Unknown operator type")
 	}
 }
 
@@ -133,9 +132,14 @@ func (it *Comparison) Next() bool {
 			return true
 		}
 	}
+	it.err = it.subIt.Err()
 	return false
 }
 
+func (it *Comparison) Err() error {
+	return it.err
+}
+
 // DEPRECATED
 func (it *Comparison) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
@@ -149,6 +153,7 @@ func (it *Comparison) NextPath() bool {
 	for {
 		hasNext := it.subIt.NextPath()
 		if !hasNext {
+			it.err = it.subIt.Err()
 			return false
 		}
 		if it.doComparison(it.subIt.Result()) {
@@ -168,7 +173,11 @@ func (it *Comparison) Contains(val graph.Value) bool {
 	if !it.doComparison(val) {
 		return false
 	}
-	return it.subIt.Contains(val)
+	ok := it.subIt.Contains(val)
+	if !ok {
+		it.err = it.subIt.Err()
+	}
+	return ok
 }
 
 // If we failed the check, then the subiterator should not contribute to the result
@@ -218,3 +227,5 @@ func (it *Comparison) Stats() graph.IteratorStats {
 func (it *Comparison) Size() (int64, bool) {
 	return 0, true
 }
+
+var _ graph.Nexter = &Comparison{}
diff --git a/graph/iterator/value_comparison_iterator_test.go b/graph/iterator/value_comparison_iterator_test.go
index b8a6cc8..ccb8c54 100644
--- a/graph/iterator/value_comparison_iterator_test.go
+++ b/graph/iterator/value_comparison_iterator_test.go
@@ -15,6 +15,7 @@
 package iterator
 
 import (
+	"errors"
 	"reflect"
 	"testing"
 
@@ -118,3 +119,17 @@ func TestVCIContains(t *testing.T) {
 		}
 	}
 }
+
+func TestComparisonIteratorErr(t *testing.T) {
+	wantErr := errors.New("unique")
+	errIt := newTestIterator(false, wantErr)
+
+	vc := NewComparison(errIt, compareLT, int64(2), simpleStore)
+
+	if vc.Next() != false {
+		t.Errorf("Comparison iterator did not pass through initial 'false'")
+	}
+	if vc.Err() != wantErr {
+		t.Errorf("Comparison iterator did not pass through underlying Err")
+	}
+}
diff --git a/graph/leveldb/all_iterator.go b/graph/leveldb/all_iterator.go
index 515133f..e0d1bdc 100644
--- a/graph/leveldb/all_iterator.go
+++ b/graph/leveldb/all_iterator.go
@@ -119,6 +119,10 @@ func (it *AllIterator) Next() bool {
 	return true
 }
 
+func (it *AllIterator) Err() error {
+	return it.iter.Error()
+}
+
 func (it *AllIterator) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
 }
@@ -141,11 +145,12 @@ func (it *AllIterator) Contains(v graph.Value) bool {
 	return true
 }
 
-func (it *AllIterator) Close() {
+func (it *AllIterator) Close() error {
 	if it.open {
 		it.iter.Release()
 		it.open = false
 	}
+	return nil
 }
 
 func (it *AllIterator) Size() (int64, bool) {
@@ -183,3 +188,5 @@ func (it *AllIterator) Stats() graph.IteratorStats {
 		Size:         s,
 	}
 }
+
+var _ graph.Nexter = &AllIterator{}
diff --git a/graph/leveldb/iterator.go b/graph/leveldb/iterator.go
index 6ccd3fe..83321ad 100644
--- a/graph/leveldb/iterator.go
+++ b/graph/leveldb/iterator.go
@@ -109,11 +109,12 @@ func (it *Iterator) Clone() graph.Iterator {
 	return out
 }
 
-func (it *Iterator) Close() {
+func (it *Iterator) Close() error {
 	if it.open {
 		it.iter.Release()
 		it.open = false
 	}
+	return nil
 }
 
 func (it *Iterator) isLiveValue(val []byte) bool {
@@ -154,6 +155,10 @@ func (it *Iterator) Next() bool {
 	return false
 }
 
+func (it *Iterator) Err() error {
+	return it.iter.Error()
+}
+
 func (it *Iterator) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
 }
@@ -283,3 +288,5 @@ func (it *Iterator) Stats() graph.IteratorStats {
 		Size:         s,
 	}
 }
+
+var _ graph.Nexter = &Iterator{}
diff --git a/graph/leveldb/quadstore.go b/graph/leveldb/quadstore.go
index 799688d..73cf386 100644
--- a/graph/leveldb/quadstore.go
+++ b/graph/leveldb/quadstore.go
@@ -90,7 +90,10 @@ func newQuadStore(path string, options graph.Options) (graph.QuadStore, error) {
 	var err error
 	qs.path = path
 	cacheSize := DefaultCacheSize
-	if val, ok := options.IntKey("cache_size_mb"); ok {
+	val, ok, err := options.IntKey("cache_size_mb")
+	if err != nil {
+		return nil, err
+	} else if ok {
 		cacheSize = val
 	}
 	qs.dbOpts = &opt.Options{
@@ -99,7 +102,10 @@ func newQuadStore(path string, options graph.Options) (graph.QuadStore, error) {
 	qs.dbOpts.ErrorIfMissing = true
 
 	writeBufferSize := DefaultWriteBufferSize
-	if val, ok := options.IntKey("writeBufferSize"); ok {
+	val, ok, err = options.IntKey("writeBufferSize")
+	if err != nil {
+		return nil, err
+	} else if ok {
 		writeBufferSize = val
 	}
 	qs.dbOpts.WriteBuffer = writeBufferSize * opt.MiB
diff --git a/graph/memstore/all_iterator.go b/graph/memstore/all_iterator.go
index 2d78ead..abfc105 100644
--- a/graph/memstore/all_iterator.go
+++ b/graph/memstore/all_iterator.go
@@ -52,6 +52,10 @@ func (it *nodesAllIterator) Next() bool {
 	return true
 }
 
+func (it *nodesAllIterator) Err() error {
+	return nil
+}
+
 func newQuadsAllIterator(qs *QuadStore) *quadsAllIterator {
 	var out quadsAllIterator
 	out.Int64 = *iterator.NewInt64(1, qs.nextQuadID-1)
@@ -69,3 +73,6 @@ func (it *quadsAllIterator) Next() bool {
 	}
 	return out
 }
+
+var _ graph.Nexter = &nodesAllIterator{}
+var _ graph.Nexter = &quadsAllIterator{}
diff --git a/graph/memstore/iterator.go b/graph/memstore/iterator.go
index d79e6c8..b39b4bf 100644
--- a/graph/memstore/iterator.go
+++ b/graph/memstore/iterator.go
@@ -15,6 +15,7 @@
 package memstore
 
 import (
+	"io"
 	"math"
 
 	"github.com/google/cayley/graph"
@@ -30,6 +31,7 @@ type Iterator struct {
 	iter   *b.Enumerator
 	data   string
 	result graph.Value
+	err    error
 }
 
 func cmp(a, b int64) int {
@@ -104,7 +106,9 @@ func (it *Iterator) Clone() graph.Iterator {
 	return m
 }
 
-func (it *Iterator) Close() {}
+func (it *Iterator) Close() error {
+	return nil
+}
 
 func (it *Iterator) checkValid(index int64) bool {
 	return it.qs.log[index].DeletedBy == 0
@@ -118,6 +122,9 @@ func (it *Iterator) Next() bool {
 	}
 	result, _, err := it.iter.Next()
 	if err != nil {
+		if err != io.EOF {
+			it.err = err
+		}
 		return graph.NextLogOut(it, nil, false)
 	}
 	if !it.checkValid(result) {
@@ -127,6 +134,10 @@ func (it *Iterator) Next() bool {
 	return graph.NextLogOut(it, it.result, true)
 }
 
+func (it *Iterator) Err() error {
+	return it.err
+}
+
 func (it *Iterator) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
 }
@@ -191,3 +202,5 @@ func (it *Iterator) Stats() graph.IteratorStats {
 		Size:         int64(it.tree.Len()),
 	}
 }
+
+var _ graph.Nexter = &Iterator{}
diff --git a/graph/mongo/iterator.go b/graph/mongo/iterator.go
index 876dff8..6409901 100644
--- a/graph/mongo/iterator.go
+++ b/graph/mongo/iterator.go
@@ -39,6 +39,7 @@ type Iterator struct {
 	constraint bson.M
 	collection string
 	result     graph.Value
+	err        error
 }
 
 func NewIterator(qs *QuadStore, collection string, d quad.Direction, val graph.Value) *Iterator {
@@ -98,8 +99,8 @@ func (it *Iterator) Reset() {
 
 }
 
-func (it *Iterator) Close() {
-	it.iter.Close()
+func (it *Iterator) Close() error {
+	return it.iter.Close()
 }
 
 func (it *Iterator) Tagger() *graph.Tagger {
@@ -137,6 +138,7 @@ func (it *Iterator) Next() bool {
 	if !found {
 		err := it.iter.Err()
 		if err != nil {
+			it.err = err
 			glog.Errorln("Error Nexting Iterator: ", err)
 		}
 		return false
@@ -148,6 +150,10 @@ func (it *Iterator) Next() bool {
 	return true
 }
 
+func (it *Iterator) Err() error {
+	return it.err
+}
+
 func (it *Iterator) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
 }
@@ -230,3 +236,5 @@ func (it *Iterator) Stats() graph.IteratorStats {
 		Size:         size,
 	}
 }
+
+var _ graph.Nexter = &Iterator{}
diff --git a/graph/mongo/quadstore.go b/graph/mongo/quadstore.go
index ce05efe..2a74be9 100644
--- a/graph/mongo/quadstore.go
+++ b/graph/mongo/quadstore.go
@@ -57,7 +57,10 @@ func createNewMongoGraph(addr string, options graph.Options) error {
 	}
 	conn.SetSafe(&mgo.Safe{})
 	dbName := DefaultDBName
-	if val, ok := options.StringKey("database_name"); ok {
+	val, ok, err := options.StringKey("database_name")
+	if err != nil {
+		return err
+	} else if ok {
 		dbName = val
 	}
 	db := conn.DB(dbName)
@@ -94,7 +97,10 @@ func newQuadStore(addr string, options graph.Options) (graph.QuadStore, error) {
 	}
 	conn.SetSafe(&mgo.Safe{})
 	dbName := DefaultDBName
-	if val, ok := options.StringKey("database_name"); ok {
+	val, ok, err := options.StringKey("database_name")
+	if err != nil {
+		return nil, err
+	} else if ok {
 		dbName = val
 	}
 	qs.db = conn.DB(dbName)
diff --git a/graph/primarykey.go b/graph/primarykey.go
index 1df1c51..5332a8f 100644
--- a/graph/primarykey.go
+++ b/graph/primarykey.go
@@ -84,8 +84,9 @@ func (p *PrimaryKey) Int() int64 {
 	case sequential:
 		return p.sequentialID
 	case unique:
-		glog.Fatal("UUID cannot be cast to an int64")
-		return -1
+		msg := "UUID cannot be converted to an int64"
+		glog.Errorln(msg)
+		panic(msg)
 	}
 	return -1
 }
diff --git a/graph/quadstore.go b/graph/quadstore.go
index e9a1a4f..8a89885 100644
--- a/graph/quadstore.go
+++ b/graph/quadstore.go
@@ -23,8 +23,8 @@ package graph
 
 import (
 	"errors"
+	"fmt"
 
-	"github.com/barakmich/glog"
 	"github.com/google/cayley/quad"
 )
 
@@ -103,40 +103,40 @@ type QuadStore interface {
 
 type Options map[string]interface{}
 
-func (d Options) IntKey(key string) (int, bool) {
+func (d Options) IntKey(key string) (int, bool, error) {
 	if val, ok := d[key]; ok {
 		switch vv := val.(type) {
 		case float64:
-			return int(vv), true
+			return int(vv), true, nil
 		default:
-			glog.Fatalln("Invalid", key, "parameter type from config.")
+			return 0, false, fmt.Errorf("Invalid %s parameter type from config: %T", key, val)
 		}
 	}
-	return 0, false
+	return 0, false, nil
 }
 
-func (d Options) StringKey(key string) (string, bool) {
+func (d Options) StringKey(key string) (string, bool, error) {
 	if val, ok := d[key]; ok {
 		switch vv := val.(type) {
 		case string:
-			return vv, true
+			return vv, true, nil
 		default:
-			glog.Fatalln("Invalid", key, "parameter type from config.")
+			return "", false, fmt.Errorf("Invalid %s parameter type from config: %T", key, val)
 		}
 	}
-	return "", false
+	return "", false, nil
 }
 
-func (d Options) BoolKey(key string) (bool, bool) {
+func (d Options) BoolKey(key string) (bool, bool, error) {
 	if val, ok := d[key]; ok {
 		switch vv := val.(type) {
 		case bool:
-			return vv, true
+			return vv, true, nil
 		default:
-			glog.Fatalln("Invalid", key, "parameter type from config.")
+			return false, false, fmt.Errorf("Invalid %s parameter type from config: %T", key, val)
 		}
 	}
-	return false, false
+	return false, false, nil
 }
 
 var ErrCannotBulkLoad = errors.New("quadstore: cannot bulk load")
diff --git a/query/mql/build_iterator.go b/query/mql/build_iterator.go
index d1469d7..8d81717 100644
--- a/query/mql/build_iterator.go
+++ b/query/mql/build_iterator.go
@@ -17,7 +17,6 @@ package mql
 import (
 	"errors"
 	"fmt"
-	"log"
 	"math"
 	"strings"
 
@@ -93,7 +92,7 @@ func (q *Query) buildIteratorTreeInternal(query interface{}, path Path) (it grap
 		it = q.buildResultIterator(path)
 		optional = true
 	default:
-		log.Fatal("Unknown JSON type?", query)
+		err = fmt.Errorf("Unknown JSON type: %T")
 	}
 	if err != nil {
 		return nil, false, err
diff --git a/writer/single.go b/writer/single.go
index 53d46fa..c62788b 100644
--- a/writer/single.go
+++ b/writer/single.go
@@ -32,18 +32,28 @@ type Single struct {
 }
 
 func NewSingleReplication(qs graph.QuadStore, opts graph.Options) (graph.QuadWriter, error) {
-	var ignoreMissing, ignoreDuplicate bool
+	var (
+		ignoreMissing   bool
+		ignoreDuplicate bool
+		err             error
+	)
 
 	if *graph.IgnoreMissing {
 		ignoreMissing = true
 	} else {
-		ignoreMissing, _ = opts.BoolKey("ignore_missing")
+		ignoreMissing, _, err = opts.BoolKey("ignore_missing")
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	if *graph.IgnoreDup {
 		ignoreDuplicate = true
 	} else {
-		ignoreDuplicate, _ = opts.BoolKey("ignore_duplicate")
+		ignoreDuplicate, _, err = opts.BoolKey("ignore_duplicate")
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	return &Single{