Gastineau Girls. One of the episodes during the end credits, it had just a quick 3 or 4 second shot of it. She doesn't laugh hysterically, but she sort of scrunches her face up or something like she's trying to keep it together.
If you get the 2-disc version of Knocked Up, amongst the many features you will find on there is "Line-O-Rama" on disc 1. Ten or fifteen seconds into it there's a brief sequence of takes in which Paul Rudd blows a raspberry on Leslie Mann's neck. If you haven't seen the movie, you might remember...