:nasal demons: n. Recognized shorthand on the USENET group
comp.std.c for any unexpected behavior of a C compiler on
encountering an undefined construct. During a discussion on that
group in early 1992, a regular remarked "When the compiler
encounters [a given undefined construct] it is legal for it to make
demons fly out of your nose" (the implication is that the compiler
may choose any arbitrarily bizarre way to interpret the code
without violating the ANSI C standard). Someone else followed up
with a reference to "nasal demons", which quickly became
established.
-- The AI Hackers Dictionary