It might be a bug. Please file a Connect bug with a repro if you have one.
In the meantime, you might be able to workaround the problem by setting the debugger options to use the legacy debug engine: Tools -> Options -> Debugging -> General: check both “Use the legacy C# and VB expression evaluators” and “Use Managed Compatibility Mode”.
That workaround has worked for me every time. But please, do still file the bugs!