Function works first time Briefly feel elation... Did it really work?
Having code work the first time is more annoying than having it fail.
If it doesn't work I go poke around to fix the failing tests, and when it finally passes I feel confident that it really works.
When code works the first time I still go poke around in the unit tests, only this time I have no idea what I'm looking for. Did it really get tested? Does it really test what I think its testing? Surely there is something wrong somewhere.