To do shitty stuff that kills everyones buzz at a social gathering is commonly referred to as 'party pooping'. 'Party toilet paper' is any act whereby one who has previously pooped on the party does something totally rad to undo said party poopery and bring everyone's spirits back up to awesomeness levels never anticipated.
me: Did you see that douchebag who pissed in the laundry hamper earlier? What a party pooper.
you: Ya, that was not cool, but then he totally made up for it when he brought that extra keg later! Party toilet paper!
(okay, maybe this term is pretty stupid)