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heisenbug

   Principle in quantum physics] n. 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.