diff --git a/packages/mongo-livedata/oplog_observe_driver.js b/packages/mongo-livedata/oplog_observe_driver.js
index 50fcd10564..098cf10eed 100644
--- a/packages/mongo-livedata/oplog_observe_driver.js
+++ b/packages/mongo-livedata/oplog_observe_driver.js
@@ -49,7 +49,11 @@ OplogObserveDriver = function (options) {
     self._unpublishedBuffer = null;
     self._published = new LocalCollection._IdMap;
   }
-  self._justUpdatedBuffer = false;
+
+  // Indicates if it is safe to insert a new document at the end of the buffer
+  // for this query. i.e. it is known that there are no documents matching the
+  // selector those are not in published or buffer.
+  self._safeAppendToBuffer = false;
 
   self._stopped = false;
   self._stopHandles = [];
@@ -203,7 +207,7 @@ _.extend(OplogObserveDriver.prototype, {
       }
 
       self._unpublishedBuffer.remove(maxBufferedId);
-      self._justUpdatedBuffer = false;
+      self._safeAppendToBuffer = false;
     }
   },
   _removeBuffered: function (id) {
@@ -241,7 +245,7 @@ _.extend(OplogObserveDriver.prototype, {
     // outside of the buffer easily.
     if (!limit || self._published.size() < limit || comparator(maxPublished, fields) > 0) {
       self._addPublished(id, fields);
-    } else if ((self._justUpdatedBuffer && self._unpublishedBuffer.size() < limit) || (maxBuffered && comparator(maxBuffered, fields) > 0)) {
+    } else if ((self._safeAppendToBuffer && self._unpublishedBuffer.size() < limit) || (maxBuffered && comparator(maxBuffered, fields) > 0)) {
       self._addBuffered(id, fields);
     }
   },
@@ -299,10 +303,10 @@ _.extend(OplogObserveDriver.prototype, {
           self._removePublished(id);
           // but it can move into buffered now, check it
           var maxBuffered = self._unpublishedBuffer.get(self._unpublishedBuffer.maxElementId());
-          if (self._justUpdatedBuffer || (maxBuffered && comparator(newDoc, maxBuffered) < 0))
+          if (self._safeAppendToBuffer || (maxBuffered && comparator(newDoc, maxBuffered) < 0))
             self._addBuffered(id, newDoc);
           else
-            self._justUpdatedBuffer = false;
+            self._safeAppendToBuffer = false;
         }
       } else if (bufferedBefore) {
         oldDoc = self._unpublishedBuffer.get(id);
@@ -315,11 +319,11 @@ _.extend(OplogObserveDriver.prototype, {
         // the buffered doc was updated, it could move to published
         if (comparator(newDoc, maxPublished) < 0) {
           self._addPublished(id, newDoc);
-        } else if (self._justUpdatedBuffer || (maxBuffered && comparator(newDoc, maxBuffered) < 0)) {
+        } else if (self._safeAppendToBuffer || (maxBuffered && comparator(newDoc, maxBuffered) < 0)) {
           // stays in buffer
           self._unpublishedBuffer.set(id, newDoc);
         } else {
-          self._justUpdatedBuffer = false;
+          self._safeAppendToBuffer = false;
         }
       } else {
         throw new Error("cachedBefore implies either of publishedBefore or bufferedBefore is true.");
@@ -467,7 +471,9 @@ _.extend(OplogObserveDriver.prototype, {
     initialCursor.forEach(function (initialDoc) {
       self._addMatching(initialDoc);
     });
-    self._justUpdatedBuffer = true;
+
+    self._safeAppendToBuffer = initialCursor.count() < self._limit * 2;
+
     if (self._stopped)
       throw new Error("oplog stopped quite early");
     // Allow observeChanges calls to return. (After this, it's possible for
@@ -634,7 +640,8 @@ _.extend(OplogObserveDriver.prototype, {
       delete doc._id;
       self._addBuffered(id, doc);
     });
-    self._justUpdatedBuffer = true;
+
+    self._safeAppendToBuffer = newBuffer.size() < self._limit;
   },
 
   // This stop function is invoked from the onStop of the ObserveMultiplexer, so