Another antidote to tricky tests or code is another pair of eyes. If I have two contradictory pieces of information that I know are both correct (the code and the test, but they disagree), I'll often grab someone else and have them take a look. If you have a spec you're working from, you can also have someone take a quick look at the spec and test and see if they agree that the test is doing what you think.
We also have code reviews for all new code and we often find tests and code that are incorrect.
It's true that the passing test can give you a false sense of security. Having someone else review things can help.