diff --git a/graph/mongo/iterator.go b/graph/mongo/iterator.go
index 8485ca9..1495eca 100644
--- a/graph/mongo/iterator.go
+++ b/graph/mongo/iterator.go
@@ -140,9 +140,9 @@ func (it *Iterator) Clone() graph.Iterator {
 
 func (it *Iterator) Next() bool {
 	var result struct {
-		Id      string "_id"
-		Added   []int64
-		Deleted []int64
+		Id      string  "_id"
+		Added   []int64 "Added"
+		Deleted []int64 "Deleted"
 	}
 	found := it.iter.Next(&result)
 	if !found {
diff --git a/graph/mongo/triplestore.go b/graph/mongo/triplestore.go
index 79fe38d..7165900 100644
--- a/graph/mongo/triplestore.go
+++ b/graph/mongo/triplestore.go
@@ -170,6 +170,25 @@ func (qs *TripleStore) updateTriple(t quad.Quad, id int64, proc graph.Procedure)
 	return err
 }
 
+func (qs *TripleStore) checkValid(key string) bool {
+	var indexEntry struct {
+		Added   []int64 "Added"
+		Deleted []int64 "Deleted"
+	}
+	err := qs.db.C("quads").FindId(key).One(&indexEntry)
+	if err == mgo.ErrNotFound {
+		return false
+	}
+	if err != nil {
+		glog.Errorln("Other error checking valid quad: %s %v.", key, err)
+		return false
+	}
+	if len(indexEntry.Added) <= len(indexEntry.Deleted) {
+		return false
+	}
+	return true
+}
+
 func (qs *TripleStore) updateLog(d *graph.Delta) error {
 	var action string
 	if d.Action == graph.Add {
@@ -193,6 +212,23 @@ func (qs *TripleStore) updateLog(d *graph.Delta) error {
 func (qs *TripleStore) ApplyDeltas(in []*graph.Delta) error {
 	qs.session.SetSafe(nil)
 	ids := make(map[string]int)
+	// Pre-check the existence condition.
+	for _, d := range in {
+		key := qs.getIdForTriple(d.Quad)
+		switch d.Action {
+		case graph.Add:
+			if qs.checkValid(key) {
+				return graph.ErrQuadExists
+			}
+		case graph.Delete:
+			if !qs.checkValid(key) {
+				return graph.ErrQuadNotExist
+			}
+		}
+	}
+	if glog.V(2) {
+		glog.Infoln("Existence verified. Proceeding.")
+	}
 	for _, d := range in {
 		err := qs.updateLog(d)
 		if err != nil {
@@ -285,6 +321,9 @@ func (qs *TripleStore) Horizon() int64 {
 	var log MongoLogEntry
 	err := qs.db.C("log").Find(nil).Sort("-LogID").One(&log)
 	if err != nil {
+		if err == mgo.ErrNotFound {
+			return 0
+		}
 		glog.Errorf("Could not get Horizon from Mongo: %v", err)
 	}
 	return log.LogID
diff --git a/writer/single.go b/writer/single.go
index 794f1a8..4a3a787 100644
--- a/writer/single.go
+++ b/writer/single.go
@@ -33,8 +33,9 @@ type Single struct {
 }
 
 func NewSingleReplication(ts graph.TripleStore, opts graph.Options) (graph.QuadWriter, error) {
-	rep := &Single{nextID: ts.Horizon(), ts: ts}
-	if rep.nextID <= 0 {
+	horizon := ts.Horizon()
+	rep := &Single{nextID: horizon + 1, ts: ts}
+	if horizon <= 0 {
 		rep.nextID = 1
 	}
 	return rep, nil