Source : Free On-Line Dictionary of Computing

can't happen
     
         The traditional program comment for code
        executed under a condition that should never be true, for
        example a file size computed as negative.  Often, such a
        condition being true indicates data corruption or a faulty
        {algorithm}; it is almost always handled by emitting a fatal
        error message and terminating or crashing, since there is
        little else that can be done.
     
        Some case variant of "can't happen" is also often the text
        emitted if the "impossible" error actually happens.  Although
        "can't happen" events are genuinely infrequent in production
        code, programmers wise enough to check for them habitually are
        often surprised at how frequently they are triggered during
        development and how many headaches checking for them turns out
        to head off.
     
        See also {firewall code}, {professional programming}.
     
        [{Jargon File}]
     
        (1996-05-10)