Heisenbugs are real
I always called bugs that disappear when you debug your program or appear at different places than expected a "Heisenbug". Sometimes I add a nice subtitle like "it knows it's being watched" :D
However I just found out that this is indeed the proper name for this kind of bugs according to the Jargon File:
Jun 8th 2011, 11:38 by Sylence.