The Jargon File (version 4.4.7, 29 Dec 2003):

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.


The Free On-line Dictionary of Computing (30 December 2018):

heisenbug

    /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.

   [Jargon File]

   (1995-02-28)