Apparent problem with asynchronous + synchronous reads

I'm working on a Unity game, and we've discovered that in some situations, EndRead (in WebSocketFrame) is not called for some data that is ready on the wire. I logged an issue with Microsoft here: https://connect.microsoft.com/VisualStudio/feedback/details/1074045/using-beginread-under-the-unity-editor-will-prevent-the-async-callbacks-from-being-invoked

Their answer, which I'm still trying to understand precisely, is that doing synchronous reads from an asynchronous callback will cause problems. It's possible that the example project I gave them isn't a good representation of what websocket-sharp does (I simplified it greatly to try to demonstrate the problem). But there's definitely an issue, either in Unity/Mono/.NET or websocket-sharp.

Start by tracing WebSocketFrame.EndRead and its BeginRead callbacks, then reproduce the reported case in Unity using the example project mentioned in the issue. Compare the behavior when synchronous reads occur inside asynchronous callbacks; done means determining whether the missing EndRead calls originate in websocket-sharp or Unity/Mono/.NET.