when he's checking in at the hotel in hawaii and sarah marshall walks up and sees him
"peter, what are you doing here?"
"i'm here to murder you"<---the face he puts when he says that cracks me up

the other one is the couples dinner scene towards the end and russell brand has to wear the hawaiian shirt she gave him, when the old man passes by in his view wearing the same shirt and does the "hey, we're twins" gesture.