Revealing mistake: When the killer attacks Sid's bodyguards, the younger one has blood on his throat before the killer slashes it.
Character mistake: During the scene in which they are all arguing whether or not film sequels are better than the original or not, someone says Sigourney Weaver's classic line from the movie Aliens: "Get away from her you bitch", and Randy makes a smart comeback and tries to correct him by saying "I think the correct line was 'stay away from her you bitch'. This is a film class, right?" Wrong - the first guy was right. This line is truly unconvincing for the Randy character, being a big movie buff and all. Jamie Kennedy decided to improvise as Joshua Jackson actually made the initial mistake by saying the wrong line, causing a different mistake. (00:16:25)
Revealing mistake: When Cici is stabbed in the sorority house, her shirt is bloody, but there is not a hole in the material. (00:33:20)
Continuity mistake: When Sidney is doing her theatre performance, when she lifts up her red veil she wraps it around her shoulders twice between shots. (00:55:20)
Revealing mistake: When Randy, Gale, Dewey and Joel are sitting on a bench in the school grounds, Gale is smoking; but no smoke comes out of her mouth when she takes a drag. (00:58:35)
Continuity mistake: When Debbie Salt is holding the knife on Sidney, the knife changes positions from time to time. (01:42:30)
Continuity mistake: When Dewey and Gale are in the classroom and as they are making out, when they realise that they are being filmed, Dewey has one side of his jacket off below his shoulder and in the other camera angle it shows that he has it perfectly on. (01:18:50)
Revealing mistake: When Mickey is flying through the air after getting shot at the end of the movie, if you look closely you can see it's a stunt double. (01:46:00)
Continuity mistake: When Gale kisses Dewey she has no more lipstick on her mouth, then in the next shot she has lipstick on.
Revealing mistake: When Mickey draws the gun at the end of the movie, a hammer is heard being pulled back. Yet the gun that Mickey is using is hammerless.
Continuity mistake: When Cici is thrown from the balcony there is debris from the window all around her (glass & window pane fragments), but if you watch when she is thrown through the window in the first place, it is only a bit of debris went that far away, not nearly as much as it is shown around her. (00:33:10)
Continuity mistake: When the killer attacks Sidney and the bodyguards in the car, he smashes through the driver's side window, leaving some of the glass still around the edges. When the killer drives off, these fragments have dissapeared and the window is totally gone. (01:22:40)
Continuity mistake: When Phil and Maureen get inside the cinema and sit down, Maureen removes her bag from her shoulders. But in a following shot of her, you can see the bag straps back on her shoulders ("You see your boyfriend anywhere?"). (00:04:10)
Continuity mistake: In the opening scene, the position of the bag straps and tee-shirt straps on Maureen's shoulders keep on changing. (00:00:55)
Plot hole: It is highly unlikely that the murderer knew Phil was going to put his ear to the stall when he heard the babbling. It is even more unlikely that the murderer is going to get him on the first stab through the stall. (Which also requires a lot of strength). We also have to assume that he spent time hanging out in the bathroom knowing Phil would go there to begin with, and that other two men with weak bladders were doing the same simultaneously forcing the victim to go to the stall to begin with. (00:07:45)
Suggested correction: The killer is incoherently whispering in a strange way in the adjacent stall to lure Phil to press his ear up against it. After stabbing him through it, the killer inspects the knife inquisitively, as if checking to see if he actually got him. While it's still not a terribly plausible scene, the killer's demeanor suggests that he encountered Phil in the restroom by coincidence and improvised the kill, rather than anticipating all of Phil's actions as part of a perfectly executed plan.
The general logistics and planning of the murder are a separate issue - because no, the murder was planned. The entry just says that it's "highly unlikely", putting it mildly, that the killer could guess the exact position Phil would pick to listen to the noise. Just a few inches up or down, left or right, make a huge difference. The killer looks at the knife admiring the results, because if he had any doubts that he got his victim, he'd be trapped in a bathroom with a screaming, wounded, angry Phil and plenty people who could come and help.
To be more clear, the correction here is that Phil had heard strange talking/whispering rather than music, which makes it at least a little more plausible the killer would think he might put his head up against the wall at a certain spot. Unlikely for sure, but unlikely isn't a mistake, it's just what movies do. Phil's death was planned yes, though it stands to reason the plan was more "surveil and strike when vulnerable" and less "wait for him in this particular stall we know he'll be next to."
Continuity mistake: In the opening scene - the premiere of Stab - when the girl is about to go back into the theater, you see the door into the theater. In the next clip, when she enters - the door is much wider. (00:06:20)
Continuity mistake: When Gale is in the theatre and gets shot and falls from the stage, she has blood stain on her left breast (above the place where the shot wound appears). Yet later, when she climbs back to the stage, she has no blood stain, only the shot wound. (01:37:20 - 01:45:45)
Continuity mistake: Shortly after the scene in which the killer pulls Randy into the van, Gale and Dewey run to the van and see the blood. Right before they open the door, there is a camera shot from inside the van through the window. You can see that the window is not broken: it is obvious that there is some sort of white fabric over it. (01:06:05)
Factual error: At the end of the film there is an LA county sticker on the ambulance, but the events are nowhere near LA. (01:52:05)
Plot hole: After Sid and her friend get out of the car, they run away from it. Yet somehow while they are talking, the killer climbs out of the car and ends up behind them without them seeing a thing.
