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).
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