Continuity mistake: In the scene where Justin arrives home from school, Richard is on his bed wearing a black hooded top and long coat. The camera angle cuts to beside the bed, and Richard is only wearing the black hoodie (it goes just to his waist), then as he stands up he is wearing the long coat again.
Continuity mistake: In the scene where Cass first goes through Carl Hudson's file, she is looking at pictures of Jane Doe. The pictures on the tabletop completely change between the first shot and the second, but the same paperwork is visible at the bottom of the screen both times so it's not two different areas.
Continuity mistake: In the scene where Richard confronts Cassie in her car outside the restaurant, she pushes him away from her window and gets out of the car. When she says, "there was a killer at your school...", we see Richard with his arms down by his side. In the next shot a split-second later, his right arm is up against the car, behind Cassie's head. We didn't see him motion in towards the car or hear the leather of his jacket moving, had he raised his arm.
Continuity mistake: Justin's pop quiz changes significantly between when he is looking at it and when Lisa shows him the completed quiz. Though the handwriting on his name stays the same (indicating that she made alterations to his actual quiz and not a new copy), the period number and answer circles both change in location and intensity.