Continuity mistake: After Chloe disappears, a boy stops Hallie and talks to her by a fence around a baseball field. In one shot the fence is unbroken behind her and they don't move from their spot, but a couple of shots later suddenly there is a gap in the fence behind Hallie. (00:44:45)
Continuity mistake: When Sean is introduced for the first time, there's an obvious continuity issue with Blake Lively's hair over her right shoulder. (00:12:00)
Continuity mistake: When Jason lunges at Batman with his knife, there is a pile of garbage by the wall where he lands. In the first shot the barrel by the garbage is wooden. But in the next shot it's suddenly a metal barrel/drum. It then changes back to a wooden barrel again. (00:04:20)
Continuity mistake: Velma reluctantly agrees to Daphne's idea to check out of the mysterious locker. She says "We'll come up with something." Right part of the frame, and in the background there's an extra (black kid with a plaid white shirt) that is simultaneously also eating at the table in the reverse shot. (00:31:15)
Continuity mistake: In the classroom when Peter is being possessed, he slams his face down into his desk twice, splattering blood onto it. After he falls back screaming, the camera changes angles and now his desk is clear of blood. (01:38:25)