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  is also often the text emitted if the impossible error
actually happens! Although  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).

