Our UI code is single threaded and UI classes use some shared static data. They work just fine, but the unit tests sometimes fail. Investigation led to threading issues. Visual Studio unconditionally runs tests in multiple threads simultaneously, and apparently there is no way to tell it not to do that.

http://social.msdn.microsoft.com/Forums/en-US/vstswebtest/thread/4cdd4668-d99c-4699-8e1c-b775498593e7