heisenbug
A software bug which fails to manifest itself or manifests differently during debugging.
I recently encountered a particularly infuriating Heisenbug. A large program, when given a large input, was just mysteriously dying. Of course, I ran it under the debugger. The mystery deepened: the program returned quietly to the debugger.
Where it comes from
(ca. 1983) Blend of Heisenberg + bug. Named for the Heisenberg uncertainty principle, which states very roughly that any attempt to measure the position of a subatomic particle will disrupt its movement, making it harder to predict.
Adapted from the Wiktionary entry heisenbug (CC BY-SA 4.0).