We had a bug once where some enemies weren't moving with their correct starting trajectories. I wrote a unit test and fixed it. Today - the test failed! And it was for real, because of a change I just made that had unforeseen consequences. If the test hadn't been there, I would have checked in, and the bug would have sat there, probably until Richard or Skaff played the problem levels and noticed - which could easily have ended up being the night before our IGF submission is due... whew.