Revealing mistake: Toward the end, after Ricky decapitates the old Mother Superior, a young nun assumes she is alive and taps her shoulder. Mother Superior's head falls off and to the floor. There is no blood whatsoever. The old woman had just been decapitated moments earlier, and there isn't even a drop of blood, not even on the white part of her nun outfit at the neck.
Visible crew/equipment: Towards the beginning, during Ricky's talk with the psychiatric doctor, there is a scene where Ricky is sitting in his chair talking, and at the top of the screen, there is a very noticeable microphone hanging down into the camera's view. A few moments afterward, you see it fly up and out of the camera's view.
Character mistake: After Ricky has just killed his girlfriend with the car antenna, he's challenged by a police officer. Officers would never walk right up to someone who have just committed a murder. Police would always order the suspect into a position that presents the least amount of threat and minimal chance of resistance. Grabbing the officer's gun would never had been an option.