In the scene where Joe pulls out a gun to shoot at his steak, it can be seen that on Brian's plate there is a steak. In the other scenes Brian's plate is empty. The steak can be seen again when Brian takes Joe's gun away.
In the beginning of the scene there are plenty of chairs, but when the man on the farthest left in front sits down there is no chair.
When Brian shows Peter his orbit, the apple passes through Peter’s hand.
When Peter goes to put the hula hoop on around him, it passes through his arms.
In the NAAFP meeting, while everybody is sitting down, Peter talks. After he is finished you see everybody getting snacks out. When the snacks are gone look at the back of the room's fat people. After Peter talks again, a few people are seen replaced by other people or sitting somewhere else.