I agree w/ pg.

The code came quickly back to me. Remember the hairy code I changed to pass the tests? Well, it was correct after all, and I broke it. The tests turned out to be wrong.

It sounds like you had missing tests. If the field testers found a bug the code tweak introduced, there was a missing test case to match whatever it was they did to uncover it. For my own part, I have found meta-tests (like for the final object state or something; more like a user would see) to uncover bugs in my code better than micro-tests (checking return values from subs/methods; more like the hacker sees).