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.