Continuity mistake: During the whole bathroom scene the trash bin by the exit door is oriented parallel to the wall or perpendicular. The mistake can already be spotted early in the scene when the cheerleader is berating Sidney, but it is much easier to notice it at the very end not just because there are multiple consecutive shots from different angles, but also because to make possible for Sid to collide with the bin as she slides across the floor, the bin has been placed almost two feet away from the wall. (00:46:30)
Continuity mistake: As Gale is talking to Dewey outside the school, when she is trying to get information from him about the killings, Dewey takes his sunglasses off and hangs them on his shirt. In the following scene they hang two different ways every time the camera cuts to him. (00:48:40)
Continuity mistake: When Dewey says "I'm 25 years old" to Gale, three people are coming down the stairs behind him. Next shot and they already went way past Gale. Same in the closing part of the scene as she says "Please, call me Gale!" and the guy above Dewey who is going up the stairs passes her by. (00:48:55 - 00:50:15)
Continuity mistake: When the principal comes back into his office after checking out who was knocking on his door you can see a closet in the background with coats in it. When they change the angle the jackets have moved to a different position. (00:50:35)
Continuity mistake: When Ghostface stabs Mr. Himbry, he is holding him by the shoulder in one and just one shot, and not in the others. (00:53:05)
Continuity mistake: Once Ghostface has stabbed Mr. Himbry, his knife points up, forward, up depending on the shot. (00:53:10)
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: In the close-up when she tells Sid not to 'freak yourself out' because they have a long night ahead, the light on Tatum is wildly different from the rest of the scene. (00:54:30)
Continuity mistake: Randy wonders with Stu if Sidney may go out with him now that Billy kinda outed himself as a killer. Stu goes through extensive Matthew Lillard-isms hunching forward, but in the next shot when he answers deadpan "No. I don't. At all." he's standing straight. (00:56:15)
Continuity mistake: When Randy shouts overenthusiastically that there's a simple formula to the murders, the extras in the background change position in the next shot when there's a whole shot dedicated to their reaction. (00:56:45)
Continuity mistake: Billy gets in Randy's face as he says he's killer. Stu puts him in a hold, but the position of his hand in the lock changes between shots. (00:57:05)
Continuity mistake: After the curfew has taken effect, Dewey and Sheriff Burke are standing on a sidewalk in town. The Sheriff is smoking a cigarette. In one shot the cigarette is almost gone, then he drops it and he steps on it, but it's now a lot longer. (00:59:40)
Continuity mistake: The sheriff and Dewey are talking; one is smoking and the other has a cone. When the sheriff says that "it's been confirmed", Dewey takes a mouthful of ice cream, but at the cut he does it again, with perfectly clean lips and without having even the time to swallow the previous. Since the joke is that he takes a mouthful each time the sheriff takes a puff of smoke, he takes one too many, one before and one after the cut. (00:59:50)
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: 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: Kenny passes the camera to Gale as she fetches the coat. The hand he used to slip her the camera joins the other hand on top of a plastic case, but in the next shot his right arm is again lowered. (01:02:45)
Continuity mistake: At the party, Randy asks (with a weird "yeeeeh!" that appears to be dubbed over) how many "hellraisers" are there. Matthew Lillard points at Rose McGowan two times in two different shots and not with the same hand either. (01:02:55)
Continuity mistake: The doorbell rings during the party and Stu tumbles over the couch to go get it; does so in two shots that are completely off when it comes to continuity, just look at his robe, and Rose McGowan's gestures with the toothpick. (01:03:15)
Continuity mistake: When Stu announces all excited that the chick from the news show is at the door, look at the necklace he's wearing, and how it changes position (barely touches his shirt's neck in close-up, it's lower in the wider angle). A few moments after, when Tatum beckons Dewey, he is even wearing it under the shirt - and Stu hasn't messed with his clothes in the meantime. (01:03:30)
Continuity mistake: When Dewey and Gale enter the house party Dewey jokingly takes a bottle of beer off a party goer then gives him it back. The bottle is back in Dewey's hand again when he speaks to Tatum. (01:03:40)
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.