Continuity mistake: When Calvin is sitting in the seat which has elastic ropes attached to it, he jumps forward to grab the grandfathers food. When he grabs the sausage, which is the last thing he eats, there is a slice of lemon on the plate, but when the grandfather looks down there is no slice of lemon. You can see this without stop motion, but it's much clearer when you do pause it.
Continuity mistake: At the hockey game, the girl has 4 full cups of beer. She gives one to Darryl and his 2 friends, and the guy sitting beside Calvin. When she turns around, there's one full cup still on the tray.