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.
Visible crew/equipment: On the DVD version, in the scene where Richard and Justin are arguing outside of the cottage near their cars, Justin slaps Richard. The shot immediately following, in which Richard is shown almost falling to the ground, you can see the microphone in the center on the bottom of the screen.
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.
Revealing mistake: In the shot where Sandra Bullock revisits the site where she was dumped years ago, you see her car pull up on what is supposed to be a moonlit road. As she gets out of her car, her shadow goes from left to right across the screen but the large tree on the left is casting a shadow in the opposite direction. As she passes the front of her car, no shadow is cast from the headlights and when she reaches the edge of the road, her shadow disappears completely.
Factual error: When the kid is trying to get the girl to come into his car, he is listening to Number of the Beast by Iron Maiden, and the editing is terrible. It jumps from the middle of the recital from the book of Revelations at the beginning to the opening guitar riff and then far to early into the famous lengthly scream by Bruce Dickinson.