Other mistake: Teddy's hair changes in every shot after being dunked in the swamp. The strap to his bag disappears and reappears as well.
Other mistake: In the treehouse scene, Teddy tells Vern that nobody will care if they didn't go to the drag race. When Teddy says this you can see Chris mouthing his lines. (00:07:45)
Other mistake: During the Lardass scene, notice that everyone is puking the same thing. There is no way that they all had pie before that.
Answer: A cello.