It is what happens when you are browsing a web page, and you accidently hit the backspace key. Can happen at any time, but is most annoying when you are watching something very important online, like the sports scores or E-Bay refreshing.
Oh look, I'm going to win the aucti-Aww, Damnit, stupid backspwnt!!
An instance where, no matter what you say afterwards, saying (absolutely) anything in defense of yourself will just make the situation much, much worse. If a brain bomb occurs, it is best to shu your trap and either
a: take the aftermath as a man, or
b: get the hell outta dodge.
Most commonly, this is done by a man to his wife, or by a man in general.
Instructor: ...What could possibly have driven this female to do such a thing?
Male student: Must have been that time of the month.