Continuity mistake: When Mort is trying to figure out what to write, he is pacing around and his hair is messed up. He walks to the chair (hair still a mess) sits down, then in the next shot there is a close up of his face and his hair is perfectly brushed.
Continuity mistake: When Mort is in Ken's office telling him about John harassing him. Ken punches the right side of the chess clock down to start it. Then Mort punches the left side, his right to stop it. Then when the camera switches back to Mort's point of view the the left side is up and the right side is down, then Ken punches down the left side to start it, when the right side should have been the one he had to press down to start it.
Continuity mistake: When Mort walked towards Amy after killing her new boyfriend, there is a clear shot of the shovel with no blood on it despite the fact he just chopped a guy through the face.
Other mistake: When Johnny Depp finds the two bodies dead in the jeep, he turns around and spots a squirrel in the tree. We get a point of view shot looking at the squirrel, before Depp faints and the camera starts falling to the left. In the next shot, however, we are facing Johnny Depp and see he has fallen on his right side.
Factual error: When Amy tries to escape from Mort in her car, there is a shot showing her feet trying to hit the right pedals to get the car moving. Her car is a SAAB 9-3, but the shot from the footwell of the car is not a SAAB.
Factual error: At the climax, Mort stabs Amy in the lower leg. She stands up and leaves a prominent bloody footprint on a stray piece of paper on the ground. Problem is, there's no way the bottom of her shoe could be covered in blood to leave the footprint. She's been bleeding for less than five seconds, and there's no pool of blood for her shoe to pick up and make the footprint with.
Continuity mistake: When Johnny Depp is searching his house for John Shooter, he opens a closet and walks away from it. During several shots you see Johnny and the closet in the background. The open position of the door keeps changing between shots.
Continuity mistake: In the scene where Mort sits at the kitchen table with his PB&J sandwich, the scene opens with a shot of the dripping kitchen faucet. Mort talks to Chico, lays on the floor and then stumbles to the sofa for a nap. Immediately following the nap, he returns to the kitchen sink and there is now a dishrag across the faucet that wasn't there in the previous shot.
Character mistake: After Mort finds the two bodies in the truck, he makes a great effort to avoid leaving any fingerprints on the door handle and also on the screwdriver he removed from the one body and takes with him. However, when he's pushing the car towards the cliff, he reaches in the window to grab the steering wheel with his bare hand, leaving his prints all over it and uses his other hand to push the truck.
Factual error: When Amy tries to escape from Mort in her car ,a SAAB 9-3, she starts the car with the gear in neutral (the car has manual gearbox). The ignition key of a SAAB can only be inserted with the gear in reverse.