Continuity mistake: Finding out that the gun is gone, Stu whines "Houston, we've got a problem here." Sid in background turns towards him instantly, but she re-turns in the next shot. (01:36:35)
Continuity mistake: Stu puts the voice box and the cellular phone inside the breast pocket of Sidney's father; the order is clear and visible even after Stu pushes the poor man to the ground; the white box is in front of the black phone. A minute later once they two-stooged their alibi we see again Mr. Prescott on the floor, and the two devices have swapped place in his pocket. (01:34:20 - 01:35:30)
Continuity mistake: It's stabbing time for the two idiot killers; Stu is about to plunge his knife into Billy. Notice Matthew Lillard's necklace as he says "I'll remember"; it's above the sweater. Cut (camera cut, that is), and it's under the sweater. Next time we see it, back to a previous angle and to the necklace being above the sweater. (01:35:10)
Continuity mistake: When Billy is taunting Sidney saying the game is called "guess how am I gonna die" he is holding the voice box in a different way between shots. (01:31:20)
Continuity mistake: When Stu and Randy are begging Sidney at the front door accusing each other, Neve Campbell's hair is pressed against her body or on top of her right arm in different shots. (01:29:10)
Continuity mistake: When Gale and Dewey nearly end up as roadkill for a bunch of teenagers who are really on a rush to go see Fonzie hanging by his entrails, Gail initially is on Dewey's right, and you can see Courteney Cox pulling David Arquette's right arm. The stuntpersons though are in a different position, the woman being on the left. (01:17:50)
Continuity mistake: When Randy pauses Halloween to launch himself into the famous "rules to survive a horror movie" speech, look at the table. There are two bottles of beer, the leftmost one apparently still with a cap. You can still see it when everyone toasts to the "no drugs and booze" rule. But as he reaches the third rule, the "I'll be right back" one, that beer is gone, and Stu is leaving to get himself a new beer. (01:13:35)
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)
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.