Recognised shorthand on the Usenet group comp.std.c for any unexpected behaviour 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.
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Nearby terms: narrowband « narrowing « NAS « nasal demons » NASI » nastistical » NASTRAN