Strangely enough, you may sometimes see an error message that says "This program has performed an illegal operation, and it will be shut down." Don't worry, it does not mean anything illegal happened; it is an incorrect use of the term and is meant to tell you that your computer or a particular program has crashed. (We wish Microsoft would reword that silly sentence; one user wrote to us after seeing that message, scared out of her wits that the feds were going to show up at her door!) Illegal also refers to "improper use" or "unauthorized access."
NetLingo Classification: Technical Terms