--- a/media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.cpp
+++ b/media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.cpp
@@ -234,17 +234,19 @@ PeerConnectionImpl::PeerConnectionImpl()
 : mRole(kRoleUnknown)
   , mCall(NULL)
   , mReadyState(kNew)
   , mIceState(kIceGathering)
   , mPCObserver(NULL)
   , mWindow(NULL)
   , mIdentity(NULL)
   , mSTSThread(NULL)
-  , mMedia(new PeerConnectionMedia(this)) {
+  , mMedia(new PeerConnectionMedia(this))
+  , mNumAudioStreams(0)
+  , mNumVideoStreams(0) {
 #ifdef MOZILLA_INTERNAL_API
   MOZ_ASSERT(NS_IsMainThread());
 #endif
 }
 
 PeerConnectionImpl::~PeerConnectionImpl()
 {
   // This aborts if not on main thread (in Debug builds)
@@ -954,31 +956,58 @@ PeerConnectionImpl::CloseStreams() {
   mCall->endCall();
   return NS_OK;
 }
 
 NS_IMETHODIMP
 PeerConnectionImpl::AddStream(nsIDOMMediaStream* aMediaStream) {
   PC_AUTO_ENTER_API_CALL(true);
 
+  if (!aMediaStream) {
+    CSFLogError(logTag, "%s: Attempt to add null stream",__FUNCTION__);
+    return NS_ERROR_FAILURE;
+  }
+
+  DOMMediaStream* stream = static_cast<DOMMediaStream*>(aMediaStream);
+  uint32_t hints = stream->GetHintContents();
+
+  // XXX Remove this check once addStream has an error callback
+  // available and/or we have plumbing to handle multiple
+  // local audio streams.
+  if ((hints & DOMMediaStream::HINT_CONTENTS_AUDIO) &&
+      mNumAudioStreams > 0) {
+    CSFLogError(logTag, "%s: Only one local audio stream is supported for now",
+                __FUNCTION__);
+    return NS_ERROR_FAILURE;
+  }
+
+  // XXX Remove this check once addStream has an error callback
+  // available and/or we have plumbing to handle multiple
+  // local video streams.
+  if ((hints & DOMMediaStream::HINT_CONTENTS_VIDEO) &&
+      mNumVideoStreams > 0) {
+    CSFLogError(logTag, "%s: Only one local video stream is supported for now",
+                __FUNCTION__);
+    return NS_ERROR_FAILURE;
+  }
+
   uint32_t stream_id;
   nsresult res = mMedia->AddStream(aMediaStream, &stream_id);
   if (NS_FAILED(res))
     return res;
 
-  DOMMediaStream* stream = static_cast<DOMMediaStream*>(aMediaStream);
-  uint32_t hints = stream->GetHintContents();
-
   // TODO(ekr@rtfm.com): these integers should be the track IDs
   if (hints & DOMMediaStream::HINT_CONTENTS_AUDIO) {
     mCall->addStream(stream_id, 0, AUDIO);
+    mNumAudioStreams++;
   }
 
   if (hints & DOMMediaStream::HINT_CONTENTS_VIDEO) {
     mCall->addStream(stream_id, 1, VIDEO);
+    mNumVideoStreams++;
   }
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 PeerConnectionImpl::RemoveStream(nsIDOMMediaStream* aMediaStream) {
   PC_AUTO_ENTER_API_CALL(true);
@@ -989,20 +1018,24 @@ PeerConnectionImpl::RemoveStream(nsIDOMM
   if (NS_FAILED(res))
     return res;
 
   DOMMediaStream* stream = static_cast<DOMMediaStream*>(aMediaStream);
   uint32_t hints = stream->GetHintContents();
 
   if (hints & DOMMediaStream::HINT_CONTENTS_AUDIO) {
     mCall->removeStream(stream_id, 0, AUDIO);
+    MOZ_ASSERT(mNumAudioStreams > 0);
+    mNumAudioStreams--;
   }
 
   if (hints & DOMMediaStream::HINT_CONTENTS_VIDEO) {
     mCall->removeStream(stream_id, 1, VIDEO);
+    MOZ_ASSERT(mNumVideoStreams > 0);
+    mNumVideoStreams--;
   }
 
   return NS_OK;
 }
 
 /*
 NS_IMETHODIMP
 PeerConnectionImpl::SetRemoteFingerprint(const char* hash, const char* fingerprint)

--- a/media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.h
+++ b/media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.h
@@ -301,16 +301,23 @@ private:
 
 #ifdef MOZILLA_INTERNAL_API
   // DataConnection that's used to get all the DataChannels
 	nsRefPtr<mozilla::DataChannelConnection> mDataConnection;
 #endif
 
   nsRefPtr<PeerConnectionMedia> mMedia;
 
+  // Temporary: used to prevent multiple audio streams or multiple video streams
+  // in a single PC. This is tied up in the IETF discussion around proper
+  // representation of multiple streams in SDP, and strongly related to
+  // Bug 840728.
+  int mNumAudioStreams;
+  int mNumVideoStreams;
+
 public:
   //these are temporary until the DataChannel Listen/Connect API is removed
   unsigned short listenPort;
   unsigned short connectPort;
   char *connectStr; // XXX ownership/free
 };
 
 // This is what is returned when you acquire on a handle

--- a/media/webrtc/signaling/src/sipcc/core/gsm/fsmdef.c
+++ b/media/webrtc/signaling/src/sipcc/core/gsm/fsmdef.c
@@ -3296,16 +3296,29 @@ fsmdef_ev_setremotedesc(sm_event_t *even
     config_get_value(CFGID_SDPMODE, &sdpmode, sizeof(sdpmode));
     if (!sdpmode) {
         ui_set_remote_description(evSetRemoteDescError, line, call_id,
             dcb->caller_id.call_instance_id, strlib_empty(),
             PC_SETREMOTEDESCERROR);
         return (SM_RC_END);
     }
 
+    // XXX We don't currently support renegotiation. If the remote SDP
+    // has already been set, return an error to the application. This is
+    // temporary until Bug 840728 is implemented.
+    if (dcb->sdp && dcb->sdp->dest_sdp) {
+        FSM_DEBUG_SM(DEB_F_PREFIX"Renegotiation not currently supported.\n",
+                     DEB_F_PREFIX_ARGS(FSM, __FUNCTION__));
+        ui_set_remote_description(evSetRemoteDescError, line, call_id,
+            dcb->caller_id.call_instance_id, strlib_empty(),
+            PC_SETREMOTEDESCERROR);
+        return (SM_RC_END);
+    }
+
+
     if (dcb == NULL) {
         FSM_DEBUG_SM(DEB_F_PREFIX"dcb is NULL.\n", DEB_F_PREFIX_ARGS(FSM, __FUNCTION__));
         return SM_RC_CLEANUP;
     }
 
     cc_initialize_msg_body_parts_info(&msg_body);
 
     /* !!! NOTE !!! The following code sets up the pointers inside