Continuity mistake: When Sidney is fighting the killer, when ever a big stunt comes up, the killer's hair gets lighter.
Continuity mistake: When Sidney is first going into the house for the end of the movie it is clear that she is not wearing the vest underneath her clothes and as she is walking around or when she is rolling on the ground fighting with her half brother you see her bellybutton but then she has the vest on and it goes over the belly button
Visible crew/equipment: When the killer throws Tyson off of the balcony, you can see the black fabric of the landing mat fly up after the stunt double lands on it. (01:20:20)
Continuity mistake: In the scene where Gail and Dewey find the killer's voice changer, Gale's hands are clean, but when it shows her hands picking up the device, there's dried blood on them. But when it shows her hands again, the blood's gone. (01:16:05)
Continuity mistake: When the killer and Sidney are locked in the screen room together and the killer slits the movie producer's throat, if you watch closely when he's talking to Sidney and waving the knife around that in the first shot there's blood on the knife and then it goes to Sidney and back on the killer and when it cuts back there is no blood on the knife. It then again goes back to Sidney, then back to the killer and the blood has magically reappeared on the knife. (01:36:00)
Continuity mistake: At the start of the movie when Cotton's girlfriend is being stalked by the killer, he stabs his knife through her bedroom door making a few large holes, especially with the first stab mark. Later, when Cotton finally gets to the house and is lookng for his girlfriend, he goes up to her door and there are now far smaller holes in the door, all in a nice pattern. When he smashes through the door, from the other side we can see there's no damage at all, then when his girlfriend emerges the stab marks are back, but in a different configuration.
Plot hole: It has already been mentioned that a certain shot reveals that Sidney isn't wearing the bullet-proof vest when she should be. Further to this, most of the shots in the climax sequence make it impossible for her to be wearing one, as you can see the contours of her chest throughout. A bullet-proof vest would flatten the appearance of this.
Audio problem: When Tyson is running down the stairs he yells something but his lips don't move. (01:19:55)
Revealing mistake: When Sarah (the one who's supposed to play 'Candy') is stabbed, her blood is dried when she slumps over the door. (00:27:10)
Continuity mistake: At the end of the movie, after Dewey asks Gale to marry him, she lays one on Dewey. She leans to the left of his face when they start kissing, and at a cut away long shot she suddenly is kissing on the right... then the close up is back at the left. (01:44:30)
Continuity mistake: In the first killing scene, the study which both Cotton and his girlfriend end up in has a white ceiling with a skylight. But when Cotton is stabbed and looks up at his killer the ceiling is light orange with beams across it.
Factual error: In the beginning of the movie, the killer calls Cotton on his cell phone. When the killer hangs up on him, you hear a dial tone. When someone cuts you off on a cell phone, you don't get a dial tone.
Continuity mistake: The first time Gale goes to Jennifer's house, if you look when Dewy gets out of his car the window is down. Look quickly to see when Dewy closes the door the window is up.
Continuity mistake: When the bodyguard finds Gale eavesdropping on Dewey and Jennifer, they soon confront each other and Tom is sitting in a chair with his left hand on the arm rest. In the next shot Tom is smoking a half finished cigarette with his left hand, but he had no time to change the hand with the cigarette in it between shots.
Continuity mistake: When the actors, Gale and Dewey are hanging around in Jennifer's house, they realise a fax is coming through. When Jennifer first approaches the machine, notice that there is actually no paper in the machine. A few shots later, the paper supply has suddenly been replenished.
Continuity mistake: When Sidney is talking to the killer in the police station, her phone has a "Nokia" sticker on the bottom of it. Later in the scene, it's gone.