Today a client coworker gave me some Java code of his to work on and finish up, because we're going to need it for forward development.
So I looked at what stuff could be refactored, wrote a couple of unit tests, refactored some bits out, ran the tests, and...
Well the code logged a few major exceptions and was unusable. But the test passed.
We were conversing about this code in general terms a few minutes later, and I pointed at the screen and hollered "I want that test to fail! Help me make it fail!"