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).