NUnit throws System.InvalidOperationException: "All test threads are blocked."

Today I went to run my NUnit tests, and found suddenly that a ton were failing! The cause of nearly all failures was the same:

System.Reflection.TargetInvocationException : Exception has been thrown by the target of an invocation. ----> System.InvalidOperationException : All test threads are blocked. Did you start new threads that the test doesn't know about?

When I Googled this exception, I found no reference of it in relation to NUnit. Has anyone seen this before?