/hi:'zen-buhg/ (From Heisenberg's Uncertainty Principle in quantum physics) A bug that disappears or alters its behaviour 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 enough that buggy code, such as that which relies on the values of uninitialised memory, behaves quite differently.)
In C, nine out of ten heisenbugs result from uninitialised auto variables, fandango on core phenomena (especially corruption of the malloc arena) or errors that smash the stack.
Opposite: Bohr bug. See also mandelbug, schroedinbug.