nasal demons

n. During a discussion on the USENET group comp.std.c 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 it 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 became recognized shorthand on that group for any unexpected behaviour of a C compiler on encountering an undefined construct.