Unusual software bug

A heisenbug is a computer bug that disappears or alters its characteristics when it is researched. One common example is a bug that occurs in a release-mode compile of a program, but not when researched under debug-mode; another is a bug caused by a race condition. The name "heisenbug" is a pun on the "Heisenberg uncertainty principle," a quantum physics term which is commonly (yet inaccurately) used to refer to the way in which observers affect the measurements of the things that they are observing, by the act of observing alone (this is actually the observer effect, and is commonly confused with the Heisenberg uncertainty principle).