Continuity mistake: In the garage scene, the position of the items by the door and switches changes without in theory anyone touching them, and does it suddenly too. For instance look at the blue board and how it is tilted towards the railing after Tatum has thrown the last bottle, or how the silver and red board is in front or behind another item. (01:05:00 - 01:07:00)
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: 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: 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: 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: 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: Once Ghostface has stabbed Mr. Himbry, his knife points up, forward, up depending on the shot. (00:53:10)
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: 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: Sidney bumps into Billy around the corner after she walked away from Tatum and Stu at the lockers. Behind Billy there's a short-haired student with a sleeveless striped top that turns the corner followed by a long-haired brunette. The two extras appear again in the next shot behind Sidney, taking almost 10 seconds to walk 2 meters. (00:43:00)
Continuity mistake: When Sidney tells Tatum to wait "Just a second" because she's going to talk to Gale outside school, Rose McGowan's bag has slipped off her shoulder during the take, but in the next shot she's wearing it properly. (00:39:55)
Continuity mistake: Sidney is in Tatum's bedroom. Tatum is playing with her rabbit toy while Sidney turns the other way. Look at the notebook, pen, ice pack. Sidney doesn't do anything but a few seconds later Tatum's mom calls her and when Sidney turns towards her the objects have changed position. (00:37:10)
Continuity mistake: When Tatum comforts Sidney telling her "I'm gonna get you out of here, okay?" in the background her brother comes out of the door holding the 'Father Death' costume box. The sheriff by him is holding an orange paper, but at the cut he's got his hand behind his back, no orange slip. (00:35:00)
Continuity mistake: When the killer pops out from the closet after Sidney closed the front door, he's holding the knife above his head, but it cuts then to a shot of him raising the hand from a position below the waist. (00:28:35)
Continuity mistake: The gang is chatting about the murders and their interrogations. Stu responds "There's no way a girl could have killed them" to Tatum's complaint that nobody asked her if she likes to hunt. In these two shots, Rose McGowan's hair is wildly different. (00:20:00)
Continuity mistake: When Tatum asks "Why would they ask you if you like to hunt?" look at the blue backpack. Next shot, and the light is on it in a different position than before. The position of the various actors as they eat grapes also changes. (00:19:50)
Factual error: At one point when the partygoers are watching Halloween, Randy alerts them about the "obligatory tit shot" and afterwards the viewers react with great satisfaction, however, in the referenced scene in Halloween, the naked breasts are actually kept out of frame.
Continuity mistake: When Billy tells Sidney that her mother is the reason his parents split, he's pointing the knife at her, but not anymore in the reverse shot. (01:33:00)
Continuity mistake: Look at Matthew Lillard once Billy says "What's the matter, Sidney? Looks like you've seen a ghost." He's scratching his cheek, but in the next shot his arm is down his side. (01:31:15)
Continuity mistake: Billy's expression is entirely different in the two shots when he raises his gun and points it at Randy. (01:30:25)
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.