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)
Continuity mistake: After Chloe runs off into the woods and the other two girls finally find her after she's had her run in with Slender Man, they are shining the flashlight in her eyes and asking her questions. In one shot her eyes are still open as she gazes near blankly off in the distance. When it cuts to a close up and suddenly her eyes are tightly squinted shut. (00:38:45)
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)