Revealing mistake: Davey at the beginning of the movie is looking at the newspaper he's delivering to Mr. Wayne Mackey. Just from a glance at the page you can tell that something is off, since the article supposedly about a missing boy appears to be written entirely in first person. Upon closer inspection, turns out that it's filler text from chapter IV of Robert Louis Stevenson' s "Treasure Island". (00:01:30)
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: In the car at the start, when Hamilton says it's like the fourth blackout today, Michael corrects her and says it's the fifth. When he says this, he is leaning forwards some as he looks up out the windshield with his mouth partly open. It cuts to a different angle and suddenly he's leaned back in his seat fully and his mouth closed. (00:01:30)
Continuity mistake: Tom and Hallie are talking in his kitchen about Slender Man after she freaked out during them trying to have sex. He backs up against the island and his left arm goes up behind his back resting on the top of it as he's blurred in the background. The angle cuts to a wide shot and suddenly his arm is down at his side. (01:01:00)
Other mistake: Even if it is a small-town police department investigating the circumstances of Don's disappearance and later discovered dead body, it is not likely that they would fail to conclude (based on forensics), or at least highly suspect, that he did not commit suicide and his death was not an accident. The police should've been able to ascertain that the angle of the gun shot in his mouth could not have been self-inflicted. Also, the blows to his forehead (from Kassi) would arouse suspicion. (01:34:01)
Continuity mistake: When Vera is in David's office, after she says "Gloria is our daughter", the shot cuts to David for a brief moment. The shot then cuts to Vera and her left hand is suddenly touching the side of her head, which she was not doing in the previous shot. (00:37:45)
Continuity mistake: Mrs. Marsh carries two plates with food to the table and starts to place the plate in her right hand in front of Mr. Marsh, but when the camera shifts, she is placing Jonathan's plate in front of him (and it appears she is doing so with her right hand, not the left hand she was holding Jonathan's in). (00:35:54)