Heisenbug

[from Heisenberg's Uncertainty Principle in quantum physics]

n. A bug that disappears or alters its behavior when one attempts to probe or isolate it. Antonym of Bohr bug; see also mandelbug. In C, nine out of ten heisenbugs result from either fandango on core phenomena (esp. lossage related to corruption of the malloc arena) or errors that smash the stack.