Revealing mistake: You can see the shadow of a camera and a microphone on Michael Biehn or Nicolas Cage back when they are fighting.
Revealing mistake: Right before Alex and Tommy are about to leave for their date, Walter is playing Tetris. When he puts down the control to come talk to them in the kitchen, the game keeps playing and you can see the pieces moving and turning.
Plot hole: In the scene in the hotel, David sees the victim's head on the floor after the killer left. Later when David is in a hospital bed he is watching TV and the reporter claims that all the heads were in Judd's car. The killer would have had to have gone back into the hotel room to get the head of that victim after David saw it. She would have put it in her bag right away if she was saving them all. The bag was even left behind. (01:12:30 - 01:25:00)
Continuity mistake: When Hexina falls to her death, she's falling face down. When a close up shot is shown, she's lying on her back.
Continuity mistake: When Susan defeats Delores Hernandez in their karate championship match, she's lying on the mat, head turned to her right. A quick shot of one of the judge and suddenly Hernandez's head is turned to her left.
Continuity mistake: Julia is apparently being helpful towards the stranger at the door. She is taking notes as he says his name and auto club card number. She is casting a visible shadow on the door, entirely gone in the close-up of the door itself. (00:08:15)