To rain on one's parade resulting in a "what the hell?" expression. Also, a non-negotiable part of an event or circumstance that ruins the rest of the event or circumstance. Term sometimes interchangeable with party foul.
T: Oh, by the way... I accidentally let Ugly Matt buy our drinks, so he's probably going to come over here and talk to us.
N: ...Poo in the 'cuzzi, dude! POO IN THE 'CUZZI.