Continuity mistake: There's a close-up of Drew Barrymore's foot when she unplugs the lights when the killer is starting his trivia. She tosses the cable and plug blindly behind her. In the next shot, the plug is closer to her foot and the shadow resulting from the change in light is also different. (00:06:25)
Continuity mistake: If you look at the close-up of Casey's hands when she locks the door, you can see her left arm in frame, but in the next shot she is holding the phone to her ear with it. Later on there's a mistake even more blatant as she unlocks momentarily the patio door to check on Steve; in close-up she uses her right hand, the killer reprimands her for the stupidity of the move, and she pulls back her left hand, as the right hand is once again the phone hand. (00:02:55 - 00:05:45)
Continuity mistake: At the police station, Billy looks back with anger at Sidney after the sheriff tells him that they have to detain him until the phone records come in. The sheriff is looking at him with his arms crossed, but in the close up on Skeet Ulrich the sheriff's got his hands on his thighs. (00:34:10)
Continuity mistake: When Sidney and Billy are talking in the bedroom at Stu's house, when the camera is on Billy, Sidney's hair is in a different position than when the camera is on her. (01:20:00)
Continuity mistake: At the end when Sidney is in Dewey's police car and the killer is there, she opens the door and falls out but the door is still locked. In some cars you can open the door when it is locked but the lock always pops up after you open the door, and this one didn't.
Continuity mistake: When Randy answers the phone he kneels down putting his head about level with the top of the TV, yet when Sidney goes to drop the TV on Stu's head the TV is much higher.
Continuity mistake: After Courtney Cox runs to the van when she's being chased, she hops into the van and throws it into reverse, guns it, then slams on the brakes, and the body falls down onto the windshield. Slamming on the brakes after going in reverse would cause the body to fall the other way, toward the back of the van. (01:22:50)
Continuity mistake: In the opening scene, Drew Barrymore put the Jiffy Pop on and it sizzled right away. In reality, it takes about ten minutes to heat up. And also she should have shaken it when it started sizzling. (00:01:25)
Other mistake: When Stu brings Sidney's dad out in the kitchen, you can see behind him a September 1996 calendar. Earlier in the movie, the scenes inside Principal Arthur Himbry's office do show a September calendar, which is obviously the intent of the movie, but it's a September 1997 calendar.
Plot hole: Given the magnitude of the crimes that occur at the house at the end, the police can be seen in the last shot dusting Dewey's truck. If the police are so careful as to dust for prints when the entire rest of the crime scene is heavily contaminated by everyone from the EMTs, Gale doing her news, police, etc. makes what they are doing seem silly. (01:43:25)
Factual error: After Gale talks to Sidney and Sidney seems doubtful of Cotton Weary's guilt, Kenny the Cameraman says to Gale, "Do you wanna go live?". Gale is an anchor/reporter for a tabloid-like show called "Top Story" (Sort of like "Current Affair", "Inside Edition", or "Hard Copy"). She does not work for CBS News or NBC News, etc. She does not have the power to "go live" and interrupt regular television programming. (00:41:55)
Continuity mistake: Sidney is on the roof trying to escape from the killer and she falls off. When she gets up she sees the dead body of Tatum in the cat door and her left arm must be behind her because you can't see it but this isn't what happened when you saw her die. Her left arm was inside the garage and was beside her. Unless if fell off, it should be next to her body.
Continuity mistake: When the cameraman is watching the 30 second delay footage and the ghost face is about to kill Randy who is on the sofa, he walks into the room in a different way on the delayed footage to on the actual film footage.
Audio problem: When Steve is on the porch and gagged, the killer instructs Casey to turn off the lights. Steve shakes his head and lets out some muffled 'No's. When the lights go out he says one last 'No' but you can still tell he isn't saying it. Even though his mouth is gagged, he makes no body movements to correlate with his shouts like he did before, especially not his jaw. Despite it being dark you can still tell. (00:06:10)
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 Drew Barrymore is running to the kitchen after the patio windows break she grabs a knife. When they show her going through the doors from the inside to go outside her knife's curving blade is pointing toward the celling but when the show her leaving the house through the doors from the outside the knife curve blade is pointing down. (00:09:25)
Character mistake: When they are surprised by Gale, Billy angrily says to Stu "Man, I thought she was dead." To which Stu replies "She looked dead, man. Still does." However, Stu didn't have time to check on Gale, since she crashed the vehicle moments before Sidney is attacked by a Ghostface that had to be Stu himself (since he does the quick clothes switch coming from the car).
Visible crew/equipment: When Drew Barrymore runs away after the killer just called her 'blondie', there's nobody behind her character, and yet a new shadow is cast on the wall to the left when the camera follows her around the corner to lock the front door. (00:03:50)
Other mistake: Two police officers drag Billy Loomis out from the sheriff office across the station; the younger officer stares into the camera right before it cuts away. (00:34:40)
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.