Revealing mistake: When Sidney sticks her finger in the wound made when Sidney stabbed Billy with the umbrella, Skeet Ulriches skin pad visibly does not connect with his neck. (01:41:30)
Continuity mistake: When the killer and Sidney are fighting by the front door, the killer takes the knife and is ready to stab Sidney. Then Sidney kicks him, and he goes flying, then watch as it goes to a wide. He is now much higher in the air than he was a moment ago. (00:27:50)
Continuity mistake: When Sidney and Tatum arrive at Stu's party, they are dropped off by Dewey. Then the two girls walk to Stu's house. The bags Tatum is carrying are switched when they are walking inside the house (when she says "Catering is here"). (01:01:30)
Continuity mistake: In the garage, Tatum slams the fridge door into the killers face, the killer knocks the door with his arm, and it opens fully. But in the next wide shot of the killer getting up, the fridge door is closed. (01:03:55)
Suggested correction: It happens 01:06:42-43 and Tatum's hand is hovering in the background, she closes it off-frame slightly.
Continuity mistake: After her first time with Billy, Sid brushes her hair. She puts the brush back on the bedstand, but the phone on it changes angle between shots. Later in the scene, you can also see the brush's handle sticking out from the side of the bedstand by the bed, or the one facing front. (01:18:55)
Continuity mistake: When Skeet is first shot by Gale he crawls backward, but if you look at the ghost face behind him there is no knife there. The next shot there is a knife on top of the ghost face.
Audio problem: When Tatum throws the first beer bottle at the killer, she yells out, "F***er," but her mouth doesn't move. (01:04:05)
Continuity mistake: After the killer calls Casey the second time she puts a salt shaker on the counter, she walks round to her living room, and the killer phones her two more times. When she gets back to the kitchen the salt shaker has disappeared.
Factual error: Gale Weathers points the gun at Billy and tries to shoot him, but the trigger doesn't pull back. This is explained by Billy when he says that the safety was on. Fine. But that gun is a Beretta 92F, and on a Beretta 92F when the safety is on, the trigger WILL pull back, but no round will fire. (01:37:20)
Continuity mistake: When all the guys are sat outside of the school talking about who (a male or a female) is the murderer, Tatum's leg is behind Stu's leg. In that same action, we go to a close up and suddenly Tatum's leg is in front of Stu's leg. (00:19:55)
Continuity mistake: When Tatum goes into the garage before she gets killed,she goes to get beer. She then walks over to the garage door opener with the beer in her right hand. Her left hand is reaching up to push the button, but as they show the close up, it is her right hand pushing the button. (01:05:35)
Continuity mistake: When Sidney is talking to Tatum and Stu by the lockers, the combination locks she's standing by are black in a closeup, but when a student runs past in a scream costume, she is by lockers with a pink and a turquoise colour lock. (00:40:50)
Revealing mistake: When Sidney discovers Tatum's body, we see her head intact, in particular the eye level. In her death scene, her cranium was being crushed right at the eyesocket. (01:07:25 - 01:22:15)
Continuity mistake: Mr. Himbry calls the two expelled students "Desensitized little sh*ts", pointing the open scissors at him. In the next shot he's looking at him but he changed position, since the scissors are gone from frame. (00:45:20)
Continuity mistake: Gale falls back on the unconscious Dewey after Billy kicks her. Her jacket is draped across the chest touching the white button of her vest. When Billy gets on his knees by her though, the jacket is fully open. (01:37:30)
Continuity mistake: Once Billy is done slashing his accomplice, he points the knife at him and tells him what to do; the knife hand keeps changing position between shots. (01:36:00)
Continuity mistake: Plummeting from the roof onto a boat, Sid causes a wetsuit (or a similar kind of fabric) to fall off. In the closer shot, the cloth is much more entangled than in the other view - look at the pink band. (01:21:55)
Continuity mistake: Off-balance on the roof as she shouts for help, Sid is hanging by the window with one hand or both depending on the shot. (01:21:45)
Character mistake: Sid deduces that Billy could be the killer because he could have used his phone call from jail to make the threatening call. However, the deduction has to be incorrect, since if it was really Billy, he could have never used the voice-changing device.
Suggested correction: This will sound pedantic (hey, it's MovieMistakes.com, we're entitled), but I don't think this is what is meant by a character mistake. Sidney isn't presenting an erroneous fact that goes uncorrected, she's explaining an idea that had crossed her mind. Billy immediately gives a plausible response and by the end of the film it's evident who'd called her. The takeaway isn't that Billy could still be the killer but that Sidney can't help but suspect him despite their reconciliation.
Other mistake: Tumbling down the ditch with Dewey, you can see that Gale loses her red pumps. In the rest of the scene though she goes to Mr. Prescott's car first and then back towards the street without fetching her shoes back. (01:17:55)
