Heisenbug /hi:'zen-buhg/ 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.

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heisenbug /hi:'zen-buhg/ 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.