Crap on a crutch is one of these--it's technically a blasphenym, but its meaning is independent of its similarity to the silly, stupid piece of street-blasphemy it can stand in for. Think about it. You've got this piece of fecal matter on a crutch--not only is it absurd (as absurd as suddenly seeing TWO Doythabans when you only expected ONE), but there is conflict inherent in it. It implies the existence of a prankster somewhere. And somewhere else there is a crippled person who is going to be very angry. And then there's going to be screaming, and I have this vision of a one-legged veteran of the Tycho-Hellenic wars swinging a big, heavy, dirty-ended stick at everyone in the room.
So ask yourself... is my epithet more or less effective than what it replaces? And how can society benefit from the creation of more deeply meaningful exclamations of displeasure? 'Crap on a crutch' indeed...