heisenbug

/hi:zenbuhg/ , n. [from Heisenberg's Uncertainty Principle in quantum
physics] A bug that disappears or alters its behavior when one attempts to
probe or isolate it. (This usage is not even particularly fanciful; the use
of a debugger sometimes alters a program's operating environment
significantly enough that buggy code, such as that which relies on the
values of uninitialized memory, behaves quite differently.) Antonym of Bohr
bug ; see also mandelbug , schroedinbug. In C, nine out of ten heisenbugs
result from uninitialized auto variables, fandango on core phenomena (esp.
lossage related to corruption of the malloc arena ) or errors that smash the
stack.