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).
[glossary]
[Reference(s) to this entry by made by: {dead code}{firewall code}{This can't happen}]