Continuity mistake: When Max is running out of the school after successfully becoming cool. We see his friend with the orange mohawk at least twice. Once hanging from a statue, and another where he's reaching out from a school bus. These two shots are really close together so there is no way that he could have moved that quick.
Continuity mistake: When the toy castle is unveiled, the drawbridge is facing towards the Christmas tree, and is the same way in pretty much all the shots it is shown. But when Fritz scoffs and says "well it's kinda interesting" he walks off towards the tree. In the shot showing him heading to the tree, the drawbridge is suddenly facing the door, perpendicular to the tree.
Continuity mistake: In the lunchroom when the boy gets tripped, a bunch of the food flies out of the bowl in the slow motion shot. When the bowl lands on Steve Guttenberg's head, it's full of food.
Continuity mistake: When Angus and Yellow reach the lake, Angus's face is tore up pretty bad, but after they cross the lake and go to the building on top of the mountain his face is almost completely free of any injuries.