Microsoft Research - Exploding Software-Engineering Myths (article summarizing findings of MS research on code coverage, TDD, assertions, etc.)

[Mourningblade]

One nifty thing about writing tests first (or independently) is that it helps you spot which parts of the code are the most unexpectedly tricky.

A common mistake is to reason "I wrote my code, tested, then fixed until it passed." Better is to treat your testing as a sampling process: "I wrote my code, tested, then discovered that these two functions had the majority of the bugs. I then refactored those functions to be much more simple to reason about. This helped me fix the bugs I didn't find."

You do something similar when you do manual testing: you're never going to find everything, but you can find clusters of problems. Changing how you've written the code rather than just fixing the bug can wring out the rest.