Nasal demons is a well-recognized humorous synonym for undefined behavior, specifically as it pertains to the C programming language.

The term originally originated from a post by John F. Woods on 2/25/92 in a discussion on the Usenet group comp.std.c. Mr Woods was attempting to emphasis the fact that undefined behavior may legally (as far as the standard is concerned) result in the compiler doing just about anything - including but not limited to "having demons fly out of your nose". The aim of the post was to make the point that one cannot put the compiler at fault for input which has no defined behavior as far as the c standard is concerned.