Continuity mistake: In the scene when Dalton's father is at work, on the blackboard behind him you can see a face from the movie Saw drawn. In the next shot, the face is erased.
Suggested correction: If by "in the next shot" you meant the scene when Josh dismisses his class, the Billy puppet face is not erased in the shots facing Josh, we just don't see the left side of the blackboard. Note, in wide shots and side shots, to the right of the Billy face we can make out the erased words "new deal" and 2 vertical squiggly lines (map-type drawing). In the shots facing Josh we still see those vertical lines and the word "deal" but the Billy face is simply offscreen, to the left.