I still remember the time when a gcc bug caused MiniSat to output UNSAT for a satisfiable problem [1]. I was the author of a SAT solver, and I was chasing my tail trying to figure out why I was getting UNSAT for a satsifiable problem. I have to admit I didn't expect it to be a gcc bug... (note: bug was found by Vegard Nossum on the CryptoMiniSat mailing list)

[1] https://www.msoos.org/2013/04/gcc-4-5-2-at-sat-competition-2...