I've structured my visual studio solution so that the WCF RIA domain services live in a separate project (assembly) from the actual web project. Today I was greeted with the following exception (via Fiddler) when trying to access an operation in the domain service.

System.ServiceModel.CommunicationException: Unrecognized message version.
   at System.ServiceModel.Channels.ReceivedMessage.ReadStartEnvelope(XmlDictionaryReader reader)
   at System.ServiceModel.Channels.BufferedMessage..ctor(IBufferedMessageData messageData, RecycledMessageState recycledMessageState, Boolean[] understoodHeaders)
   at System.ServiceModel.Channels.BinaryMessageEncoderFactory.BinaryMessageEncoder.ReadMessage(ArraySegment 1 buffer, BufferManager bufferManager, String contentType)
   at System.ServiceModel.Channels.MessageEncoder.ReadMessage(ArraySegment`1 buffer, BufferManager bufferManager)
   at BinaryMessageFiddlerExtension.BinaryInspector.GetWcfBinaryMessageAsText(Byte[] encodedMessage)
   at BinaryMessageFiddlerExtension.BinaryInspector.UpdateView(Byte[] bytes)

I couldn't find any resources explaining the solution to this issue, but looks like few people encountered it. In my scenario, the error was caused because the client could not reach the service, and it was returning 404 (thanks to Fiddler), but the exception details on the client side were completely misleading.

The solution to my issue is just to add a project reference to my external assembly which contains all of my WCF RIA services.