Now, I think Mockito -- the Java mocking framework -- has taken over the mantle of "King of error messages". Earlier today I had a new unit test I had just written fail. I didn't have to dig too much to find the error because Mockito pointed the way with the following stacktrace.
Unifinished stubbing detected!
E.g. toReturn() may be missing.
Examples of correct stubbing:
Also make sure the method is not final - you cannot stub final methods.
How cool is that! Hats off, Mockito-team. This developer appreciates the extra few minutes you took to put together that message. It helped me immensely.