Continuity mistake: In the scene at the TV truck, the killer's knife is blood-free even after slicing the cameraman's throat and stabbing Sidney, if you look at him when he tries to open the door and when he gets in. When he shakes his fist at Sidney for running away though, the knife is all bloodied up. (01:24:00)
Continuity mistake: When the killer slits Kenny's throat, Sidney in the background stifles a shout with one hand on her mouth, both hands in the next shot. She opens her hands but her right hand is still glued to her face in a further cut. (01:23:40)
Continuity mistake: When Kenny the cameraman spots the front door open and turns around to look again at the truck, Sidney before the camera cuts away is turning all the way to look at the monitor as well, but in the next shot she's still facing Kenny. (01:23:35)
Audio problem: When Sidney gets in the truck, Kenny tells her "There's a camera in the house" but his mouth barely moved. (01:23:15)
Other mistake: Randy answers the phone and receives the news that the principal has been killed and left to hang from the goalpost. He gets the call from the telephone by the TV, and he his head reaches about half the height of the TV. The hidden camera is on top of the VHS player, a shelf underneath the TV, but somewhat Gale and Kenny in the TV truck get a close-up of his mouth. (01:16:00)
Continuity mistake: Randy pauses the movie and gets up to explain "the rules." Stu comments "Have an aneurysm, why don't ya?" Matthew Lillard's hand is in a different position in that shot. Not only that; the only extra you can partially see on the right in the background, in the previous shot made a disappointed gesture and lowered his arm, and in this new shot he lowers it again anew. (01:13:05)
Continuity mistake: When Sid and Billy make out and talk about Meg Ryan and other kinds of movies, their hands go on and off the back of their heads between shots. (01:11:00)
Continuity mistake: The killer is holding the knife he uses to cut Tatum at an angle that changes between shots when he's threatening her. (01:06:25)
Continuity mistake: In the garage scene, Tatum grabs 5 bottles of beer. She initially has 4 of them in her right hand, a single one in her left, and in fact she can easily reach the fridge door and close it with that hand. New shot, and she is not closing the door anymore with her left hand, suddenly busy with the other bottles, and she has to use her foot. (01:05:05)
Continuity mistake: When Gale makes her entrance in the room with all the teenagers at the party, look at her position in particular in relation with Matthew Lillard; she wouldn't pass Sidney by in the spot when it happens, and she approaches him in different positions. (01:03:50)
Continuity mistake: Gale and her crewmember just arrived at the party location. He asks if they have been spotted, she says no, and he then sinks back into the van to fetch the equipment. The shot changes and he completely turned around. (01:02:05)
Continuity mistake: On Sidney's porch, Tatum tells her that maybe her mother was having in fact an affair; in that wide angle her left pigtail is behind her shoulder, but in the close-up it is in front. (00:53:30)
Continuity mistake: The costume of Ghostface/Father Death is stuffed inside the plastic bag in two different ways in the shots when Sheriff Burke is checking it out - look at the bits coming out in the close-up. (00:35:05)
Continuity mistake: Billy holds Sidney in his arms after the attempt at her life downstairs. She clings to him and pulls at his jacket, that is unperturbed in the close-ups on Skeet Ulrich. (00:29:55)
Continuity mistake: When Sidney manages to kick Ghostface away after he has almost stabbed her behind the front door of her house, she sets up the kick shouting "no" and shielding herself with her left arm, but in the shot of the actual kick the stuntwoman's left arm is on the ground. (00:28:55)
Continuity mistake: In class, the teacher tells Sidney "It appears to be your turn." One of her classmates, a girl with a grunge shirt, turns around to look at her, but in the next shot she's looking ahead. (00:18:55)
Audio problem: Casey's dad tells his wife to call the police. Meanwhile, he runs upstairs yelling "Casey, are you upstairs?" He reacts to his wife's line turning towards her before she says "Casey, baby?" (the mother does whisper "My God" a moment before he stops and turns too, but it was a whisper, good luck hearing that when he was rushing upstairs and shouting his line). (00:12:10)
Continuity mistake: In the close-up when she clutches Ghostface's mask, Casey's hand is significantly bloodier than in the rest of the scene and the sleeve that was creased down straightened itself, with a different bloodstain. (00:11:35)
Continuity mistake: When Casey pops up to peek at the window and is face to face with the killer, she shakes with fear reacting very naturally, but in its close-up her left arm is sticking out perfectly steady and firm in an incredibly unnatural position, blatantly there holding still just so the killer can grab it. (00:10:20)
Continuity mistake: When Casey peeks at the letter opener on top of the TV, her hair is on top of the cordless phone; in the shots from the side, before and after, her wig is pressed against her cheek and ear by the phone itself. (00:08:40)
Answer: Landline phones like the one used in the movie cannot dial out a phone number while the line is active. Considering the killer is continuously calling her, she can't dial out on the phone line. This is referenced in the movie itself when her parents arrive home, attempt to dial 911, and can't due to the line still being active.