8. Heisenbug Source: Unknown A computer bug that disappears or alters its characteristics when an attempt is made to study it.
On the other note - I personally think most online communities tend to get more lenient overtime. I'd be interested in any further reading on that matter.
Rob Pike mentioned this: http://commandcenter.blogspot.ro/2010/08/know-your-science.h...
Timing-related errors make the worst heisenbugs. I had one once where pausing through the code caused the code to work, so I cobbled together a quick debug logging facility for it, and that still caused the code to work just fine. Anything that altered the code by more than a few milliseconds of execution time caused it to work OK.
I stared at it for a while and then rewrote the entire block in disgust.
I know one multiplayer Flash service has a problem where their server clocks had drifted apart, and it caused all sorts of crazy issues. Took them a couple of weeks to figure it out and fix the problem. Not quite a Heisenbug maybe, but close. (http://playerio.com/forum/multiplayer/room-disappears-from-l...)
I've had bugs before that have seemed to come and go for absolutely no reason - I think they involved corrupted incremental compilations, but who knows.