(at around 34 mins) When Gale is attempting to enter the police station with Kenny the cameraman, she is stopped by a police officer and is heard saying "Hey watch the hand, do you know who you're dealing with here?!" But her mouth isn't moving.
(at around 31 mins) When the police arrest Billy after Sid's first attack, they place him in the car and close the car door, in which the windows are closed. After they've driven off, Sheriff and Dewey go to check on Sid and in the background we can still hear Billy calling out for Sid crying out "SID!"
(at around 1h 29 mins) When Stu and Randy are pleading with Sidney, she decides to trust neither and locks them both out. At that point, she would've known she could trust Randy, because she saw the live feed of the killer about to attack him.
(at around 1h 12 mins) At Stu's party, everyone watches Halloween. When they watch the scene when Michael Myers stabs somebody and nails him to the wall with his knife, someone complains that the blood in that scene is too red. However, this scene had no blood, however since they are drinking alcohol and are obviously intoxicated he couldn't have known.
(at around 1h 30 mins) Billy states to Sidney that it's scarier when killers have no motive, and that Norman Bates did not have a motive for his killings. But that is incorrect; Norman Bates' motive was that he was tormented by his mother as a child, and took on her persona after murdering her.
(at around 1h 15 mins) Randy says "Here comes the obligatory tit shot" while watching Halloween and everyone cheers, supposedly at the sight of breasts, but the scene they are watching at that precise moment doesn't show nudity. The girl's breasts in Halloween aren't revealed until a later scene (and, some claim, not at all in the video version).
(at around 1h 13 mins) When Randy and friends are watching horror movies, Randy says, "She always outsmarted the killer during the big chase seen. Virgins always do that." However, when Kenny and Gale watch the footage on their hidden camera, the sentences have been shortened to, "The virgin always outsmarts the killer in the big chase scene."
(at around 1h 35 mins) When Stu is talking to Sidney at near the end of the movie, he hands the phone to Billy, who gets mad and throws it back at Stu. The phone is shown that it hits him and falls to the ground but when Stu picks it back up to talk on it, its on the counter and easier for him to pick up.
(at around 6 mins) When Casey sees Steve on the patio, the killer tells her to turn off the light. You see the shadow fall across his face. Yet when the camera flashes to Steve just before he's killed, we see the already turned-off light go off again.
When Gale and Kenny are watching the footage in the news van, the lines that Randy and Stu are saying and the way they are being said are different than when it took place. It also happens when Kenny and Sidney are watching the footage of Randy talking to the TV during "Halloween" when the killer is about to stab him.
(at around 1h 5 mins) When Tatum is getting beers from the refrigerator in the garage, she is seen grabbing beers and holding them in her right arm. When she's done, she is originally seen from behind holding a beer in her left hand, and closing the door with her forearm, but then it switches shots, and she's seen holding two beers with her hand tucked into her arm, and closing the refrigerator door with her foot.
(at around 11 mins) After Casey is stabbed the first time she knocks the knife out of the killer's hands, causing him to start strangling her with both hands. When she fights him off, the knife is suddenly back in his hand again.
(at around 1h 5 mins) When Tatum is locked in the garage she reaches up to hit the garage door button to get out with her left hand because the beer bottles are in her right. On the close-up of her hand pushing the button it's her right that hits it.
(at around 1h 19 mins) When Sidney and Billy are up in Stu's bedroom after they have sex, Sidney brushes her hair all to one side. Her hair then proceeds to change completely from scene to scene as they talk.
(at around 32 mins) When Tatum arrives at Sidney's house after the first failed attempt on Sidney's life, Tatum is clearly wearing white pantyhose. But later when she arrives at the police station to meet Sidney, she is still wearing the same outfit but without the pantyhose. Theoretically, she could've gone home or somewhere else off camera and took them off, but we see Tatum get into her car at Sidney's house to presumably follow right behind Dewey's patrol car to the station.
(at around 1 min) When Casey is making the popcorn, she starts pulling the cover off with her left hand near the handle. In the next shot she is seen using her right hand pulling the paper cover from the other side.
(at around 36 mins) When Sidney punches Gale in the face, she falls and Kenny the cameraman catches her. In the first shot, Kenny catches Gale with his hand hooked under her armpit. Immediately after the jump-cut, it shows him with his hand on the outside of her arm, just below the shoulder.
(at around 49 mins) When Gale and Dewey are talking outside the school, Dewey folds up his sunglasses and hangs them on his shirt. In the next scene they are upside down and then seconds later back in the original position.
(at around 1h 7 mins) When Tatum is throwing the beer bottles at Ghostface, one clearly hits him hard in the forehead. Yet, moments later we see both the possible killers and neither of them even has a mark on their faces.
(at around 1h 27 mins) When Sidney locks herself in Dewey's Jeep, the door locks are shown to be on top of the door panel, but are not shown in the outside view of the SUV. Also the locks are designed next to the door handle, meaning the locks shown are from a different vehicle.
(at around 1h 26 mins) When Gale is driving the news van, she brakes suddenly while in reverse, causing the cameraman's body to fall onto the windshield. Since she was going backward, inertia would have cause the body to continue going backward, falling instead from the back of the vehicle.
(at around 13 mins) At the beginning when Billy shows up at Sidney's window he climbs it. When he leaves he simply walks away when her bedroom is supposedly on the first floor of the house and not on the ground floor.
(at around 55 mins) When Stu scares Randy in the video store he drops a video case which is obviously empty. However, in order to prevent theft, most video stores displayed empty cases to the public. When a customer wanted to rent a video they would take the empty case to an employee who would then retrieve the tape for them.
(at around 9 mins) When Casey runs into the kitchen after her boyfriend Steve is killed, the smoke alarm sounds like a fire truck siren. Much later, when Casey's mother and father come into the kitchen, the smoke alarm is making a "beeping" sound. This because many smoke alarm will do that when the battery gets weak. After whaling the full siren alarm, the battery got weak and the sound changed to the beeping sound.
(at around 51 mins) When the principal goes to open the door the first time. The door opens left to right (the hinges are on the right side.) When he closes it he closes it left to right (the hinges are on the left side). However, it is revealed that in this shot we are looking at a mirror when the principal turns around and is startled by his reflection, then proceeds to fix his hair.
(at around 29 mins) When Sidney is attacked in her home, she kicks Ghostface before he can stab her, tries to open the front door, and then proceeds up the stairs. Ghostface chases her up the stairs and, if you listen closely, you can hear a voice saying "Easy! Easy!"
(at around 1h 35 mins) At the end of the movie when Gale Weathers is holding the gun in the kitchen, you can clearly hear the sound of the hammer being pulled before the camera goes to her. However, when the camera shows a view from behind, you can see the hammer on the gun is still up and not in a firing position. Also, on the standard Beretta 92FS handgun, the hammer is unable to cocked back with the safety on, as Billy stated it was. Furthermore, when the safety is on, the trigger will pull freely. When Gale tried pulling the trigger, she encountered resistance.
(at around 1h 40 mins) At the end of the film, Gale is holding a microphone while reporting the news, but as the camera zooms back a person is holding a boom microphone, which is not needed when using a hand-microphone.
(at around 10 mins) When the killer is chasing Casey outside her house, the part of the mask that hides his neck is grey. But when the killer is strangling Casey on the grass, the neck part is jet-black and the face of the mask is also wider.
(at around 1h 30 mins) When Stu is setting up Sidney's dad to be framed, you can see him put the cell phone in front of the voice changer box in his pocket. But when it cuts back and her dad is pushed to the floor, you see the cell phone is now behind the voice changer box.
(at around 1h 40 mins) Near the end, when Sidney's dad crashes through the door tied up, you can see a crew member pushing mops/brooms etc out after he falls (better spotted when paused and pressing frame advance button).
(at around 1h 14 mins) Gale tells Kenny "I'll be right back," but survives the trilogy. We are specifically told that any character who uses this line will be killed. However, the idea of the Scream Series was to break the rules and clichés of slasher films, which is what it achieved with her surviving despite breaking one of the rules.
(at around 1h 20 mins) Many view it as a continuity error when Billy gets "stabbed" by the killer, and it shows the killer wiping off the knife, but right after it shows blood still on the knife. Then he wipes it off again. When watched closely, it is revealed that the first wipe is actually the killer wiping the fake blood onto the knife before making a show of wiping it off in front of Sidney.
The film never appears to really give an explanation for why Casey and Steve were murdered. Stu had motivation because Casey had dumped him for Steve. Billy also tells Sidney that it's a lot scarier when there is no motive.
(at around 10 mins) When Casey is outside, the killer comes from behind and stabs a knife into her chest, when his hand goes back to stab her for the second time you can see that the knife is a prop with a retractable tip.