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.
Continuity mistake: While Ricky is on his shooting spree, he manages to shoot at an oncoming car, causing it to to crash and roll over. As the car rights itself, the driver and the car seats are visible. The right rear passenger window is also fully intact. Just before the car explodes, the driver has disappeared, along with the car seats and the same rear side window is now completely missing.
Continuity mistake: When the mother superior enters the kitchen to retrieve a knife to protect herself, there is a coffee mug on the bench in front of a white vase. When she turns around to exit the kitchen, the vase is visible, but there is now no coffee mug. Even at a different angle the mug should still be visible as well.
Plot hole: The police are on the trail of a known psychotic and mass murderer. When they turn up at the house of the mother superior, it's evident Ricky has been there, as the door has been cut open and kicked in. There is no way the other nun would have been able to freely enter the house without police escort, or even get out of the police car, as Ricky had not been located and apprehended. The house is a potential crime scene and she would not have been allowed in.
Continuity mistake: When Ricky and the mother superior are at the top of the stairs, in closeup Ricky raises the axe right up over his head ready to strike and he has one hand at the bottom of the handle and the other hand further up the handle. Cut to the next scene where the mother superior is ready to leap from her wheelchair, and Ricky is now suddenly backed into the corner. With no room for the axe to be behind him, his arms are no longer raised and his hands are now together, both at the bottom of the axe handle.
Plot hole: In the flashback scene from Part 1 when the two cops go into a house after seeing a Santa Claus climbing into a window who turned out to be a dad visiting his daughter, we hear Ricky say "Daddy almost got his present early," how would Ricky know about this scene?
Other mistake: When Ricky confronts the mother superior in the dining room, Ricky raises the axe above his head and brings it straight down in one single motion. Both Ricky and the mother superior are facing each other. At that angle, it would have been impossible for Ricky to decapitate her. At best her head would have been cleaved in half, but not decapitated.