flat_chest and trap should be mutually exclusive tags, for the flat_chest wiki says the tag only applies to girls. Should the hundred-odd posts with this combination be fixed?

That sounds perfectly reasonable and logical. Unless the male character has man-boobs they'd all have flat chest's by definition. Though (personally) I think Watarase Jun should be an exception, while being the most infamous trap can't help by feel that she is really a woman trapped in a man's body ^^

FWIW, I don't think they should be mutually exclusive, given that most of the traps we have here are impossible to distinguish from girls visually. So as long as they look like girls, they are girls for the purpose of tagging.