Heisenbugs: When you test for them they don't appear anymore. (Add a printf, et voila the race condition is gone). - I've personally experienced this one. Race condition/deadlock between 3 different threads. One mutex was inside the debug function :)
Schrödinbug: The code should/could have never worked in the first place, but did. Once you saw the code, and realized it shouldn't work, it stops working.