I had a co-worker many years ago that gave me some really frustrating (and insightful) advice on troubleshooting:

First, check all your explicit assumptions. Then, check all of your implicit assumptions.

Obviously, the second one is almost impossible. I find that writing unit tests (and TDD) helps to change some of the implicit assumptions into explicit assumptions. To me, this seems to be a benefit. (Of course, that might just be another implicit assumption.<grin/>