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.
All female students in the room: *Gasp*
Male student: Damnit, brain bomb.