In the late 90s I had an app ported to multiple unixes. We were having intermittent problems with our HP/UX port which was caused by a mutex being unlocked by

In the late 90s I had an app ported to multiple unixes. We were having intermittent problems with our HP/UX port which was caused by a mutex being unlocked by a different thread than which had locked it. In that case (which only happened under heavy worker contention) unlock happily returned an error and the mutex was left locked. Be careful about "can't possibly fail".