Continuity mistake: When Elliot's the basketball player, when he scores the slam-dunk and breaks the headboard, he breaks the headboard after the slam as he bounces/hangs off the headboard. But in the replay he breaks the headboard as he makes the slam and then bounces/hangs off the headboard.
Trivia: Not a mistake but something clever. When the Devil was a schoolteacher, the first equation on the board was Fermat's Last Theorem (X to the Nth + Y to the Nth = Z to the Nth for N is greater than 2). It was proven in 1995 that there is no integer solution to this equation; exactly the kind of problem the Devil would give to an Algebra class, but not really expected in your standard Hollywood film...