Source : Free On-Line Dictionary of Computing

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
        significantly 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
        lossage related to corruption of the malloc {arena}) or errors
        that {smash the stack}.
     
        Opposite: {Bohr bug}.  See also {mandelbug}, {schroedinbug}.
     
        [{Jargon File}]
     
        (1995-02-28)