diff --git a/ipc/chromium/src/chrome/common/ipc_message.cc b/ipc/chromium/src/chrome/common/ipc_message.cc
index 6e8c67f5ae0..783ac36130c 100644
--- a/ipc/chromium/src/chrome/common/ipc_message.cc
+++ b/ipc/chromium/src/chrome/common/ipc_message.cc
@@ -38,6 +38,7 @@ Message::Message(int32 routing_id, uint16 type, PriorityValue priority)
 #if defined(CHROMIUM_MOZILLA_BUILD)
   header()->rpc_remote_stack_depth_guess = static_cast<size_t>(-1);
   header()->rpc_local_stack_depth = static_cast<size_t>(-1);
+  header()->seqno = 0;
 #endif
   InitLoggingVariables();
 }
diff --git a/ipc/chromium/src/chrome/common/ipc_message.h b/ipc/chromium/src/chrome/common/ipc_message.h
index 4bff8f8d5d3..f5d173ed83a 100644
--- a/ipc/chromium/src/chrome/common/ipc_message.h
+++ b/ipc/chromium/src/chrome/common/ipc_message.h
@@ -142,7 +142,7 @@ class Message : public Pickle {
 
 #if defined(CHROMIUM_MOZILLA_BUILD)
   size_t rpc_remote_stack_depth_guess() const {
-      return header()->rpc_remote_stack_depth_guess;
+    return header()->rpc_remote_stack_depth_guess;
   }
 
   void set_rpc_remote_stack_depth_guess(size_t depth) {
@@ -151,13 +151,21 @@ class Message : public Pickle {
   }
 
   size_t rpc_local_stack_depth() const {
-      return header()->rpc_local_stack_depth;
+    return header()->rpc_local_stack_depth;
   }
 
   void set_rpc_local_stack_depth(size_t depth) {
     DCHECK(is_rpc());
     header()->rpc_local_stack_depth = depth;
   }
+
+  int32 seqno() const {
+    return header()->seqno;
+  }
+
+  void set_seqno(int32 seqno) {
+    header()->seqno = seqno;
+  }
 #endif
 
   template<class T>
@@ -275,6 +283,8 @@ class Message : public Pickle {
     size_t rpc_remote_stack_depth_guess;
     // The actual local stack depth.
     size_t rpc_local_stack_depth;
+    // Sequence number
+    int32 seqno;
 #endif
   };
 #pragma pack(pop)
diff --git a/ipc/glue/RPCChannel.cpp b/ipc/glue/RPCChannel.cpp
index a38c27f9356..e6551a3f53b 100644
--- a/ipc/glue/RPCChannel.cpp
+++ b/ipc/glue/RPCChannel.cpp
@@ -96,9 +96,10 @@ RPCChannel::Call(Message* msg, Message* reply)
         return false;
     }
 
-    mStack.push(*msg);
+    msg->set_seqno(NextSeqno());
     msg->set_rpc_remote_stack_depth_guess(mRemoteStackDepthGuess);
-    msg->set_rpc_local_stack_depth(StackDepth());
+    msg->set_rpc_local_stack_depth(1 + StackDepth());
+    mStack.push(*msg);
 
     mIOLoop->PostTask(
         FROM_HERE,
@@ -137,7 +138,7 @@ RPCChannel::Call(Message* msg, Message* reply)
             continue;
         }
 
-        NS_ABORT_IF_FALSE(recvd.is_rpc(), "wtf???");
+        RPC_ASSERT(recvd.is_rpc(), "wtf???");
 
         if (recvd.is_reply()) {
             RPC_ASSERT(0 < mStack.size(), "invalid RPC stack");
@@ -146,7 +147,9 @@ RPCChannel::Call(Message* msg, Message* reply)
 
             // FIXME/cjones: handle error
             RPC_ASSERT(
-                recvd.type() == (outcall.type()+1) || recvd.is_reply_error(),
+                recvd.is_reply_error() ||
+                (recvd.type() == (outcall.type()+1) &&
+                 recvd.seqno() == outcall.seqno()),
                 "somebody's misbehavin'", "rpc", true);
 
             // we received a reply to our most recent outstanding
@@ -355,6 +358,8 @@ RPCChannel::DispatchIncall(const Message& call)
         reply->set_reply_error();
     }
 
+    reply->set_seqno(call.seqno());
+
     mIOLoop->PostTask(
         FROM_HERE,
         NewRunnableMethod(this, &RPCChannel::OnSend, reply));
diff --git a/ipc/glue/SyncChannel.cpp b/ipc/glue/SyncChannel.cpp
index 77e89e7ea15..b67bbca793f 100644
--- a/ipc/glue/SyncChannel.cpp
+++ b/ipc/glue/SyncChannel.cpp
@@ -58,7 +58,8 @@ namespace ipc {
 SyncChannel::SyncChannel(SyncListener* aListener)
   : AsyncChannel(aListener),
     mPendingReply(0),
-    mProcessingSyncMessage(false)
+    mProcessingSyncMessage(false),
+    mNextSeqno(0)
 {
   MOZ_COUNT_CTOR(SyncChannel);
 }
@@ -81,6 +82,8 @@ SyncChannel::Send(Message* msg, Message* reply)
                       "violation of sync handler invariant");
     NS_ABORT_IF_FALSE(msg->is_sync(), "can only Send() sync messages here");
 
+    msg->set_seqno(NextSeqno());
+
     MutexAutoLock lock(mMutex);
 
     if (!Connected()) {
@@ -89,6 +92,7 @@ SyncChannel::Send(Message* msg, Message* reply)
     }
 
     mPendingReply = msg->type() + 1;
+    int32 msgSeqno = msg->seqno();
     mIOLoop->PostTask(
         FROM_HERE,
         NewRunnableMethod(this, &SyncChannel::OnSend, msg));
@@ -117,8 +121,9 @@ SyncChannel::Send(Message* msg, Message* reply)
 
     // FIXME/cjones: real error handling
     NS_ABORT_IF_FALSE(mRecvd.is_sync() && mRecvd.is_reply() &&
-                      (mPendingReply == mRecvd.type() ||
-                       mRecvd.is_reply_error()),
+                      (mRecvd.is_reply_error() ||
+                       (mPendingReply == mRecvd.type() &&
+                        msgSeqno == mRecvd.seqno())),
                       "unexpected sync message");
 
     mPendingReply = 0;
@@ -151,6 +156,8 @@ SyncChannel::OnDispatchMessage(const Message& msg)
         reply->set_reply_error();
     }
 
+    reply->set_seqno(msg.seqno());
+
     mIOLoop->PostTask(
         FROM_HERE,
         NewRunnableMethod(this, &SyncChannel::OnSend, reply));
diff --git a/ipc/glue/SyncChannel.h b/ipc/glue/SyncChannel.h
index f00026fe083..3519c2d7a6d 100644
--- a/ipc/glue/SyncChannel.h
+++ b/ipc/glue/SyncChannel.h
@@ -105,9 +105,17 @@ protected:
         return mPendingReply != 0;
     }
 
+    int32 NextSeqno() {
+        AssertWorkerThread();
+        return mChild ? --mNextSeqno : ++mNextSeqno;
+    }
+
     MessageId mPendingReply;
     bool mProcessingSyncMessage;
     Message mRecvd;
+    // This is only accessed from the worker thread; seqno's are
+    // completely opaque to the IO thread.
+    int32 mNextSeqno;
 
     static bool sIsPumpingMessages;
 };