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).