Revealing mistake: As Mac and Dennis are wheeling themselves through the mall, they pass by a clothing store where a man and woman are standing by the entrance. The woman notices the camera and waves at it. (00:15:22)
Revealing mistake: Right before Mac and Dennis start to fight in the mall, people in the store stare at the camera and a woman poses at the camera.
Revealing mistake: During the training montage, right as Mac is about to smash the bottle over Charlie's head, you can see Charlie stop what he's doing about a half-second before being hit, and brace himself for the impact, despite not being able to see it coming. This is confirmed on the DVD commentary.
