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can't happen

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 (sense 2).