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)
Other mistake: It is highly unlikely that police, investigating the dead body (of a man reported missing) with a gun shot in the mouth and cut forehead, would simply back off after asking a mother if they could talk to her sixteen-year-old son (and son of the victim) and she said "no." (01:40:35)
Other mistake: Don forcefully threw Kassi against the wall. Kassi's forehead hit the wall quite hard and she was unconscious, but she had no obvious injury (bump) to her forehead. (01:34:01 - 01:36:27)
Other mistake: When Tyler is in the bathroom brushing his teeth, he is facing a wall-sized vanity mirror. The mirror should not be able to reflect the window along the adjacent wall (along the right side). Also, regardless of what angle of Tyler the mirror is reflecting, it reflects the same image of the wall. (00:30:26)
Continuity mistake: The clothes Tyler and Kassi were wearing at Kassi's house were not the same when Kassi came to dinner that day at the Burnside's house. It seems unlikely that BOTH would change clothes before dinner. (00:31:49 - 00:32:40)
Suggested correction: Apparently they did. It was probably Tyler's idea, presenting Kassi in the best possible light to assuage suspicion and maintain his image as a good Christian Boy Scout.