(at around 1h 7 mins) When Tatum throws the beer bottles at the killer in the garage, they break and wet his costume, but when the killer runs and falls over her, his costume is suddenly dry.
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 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 3 mins) When Dewey walks into the party, he takes a beer bottle from a teenager, he then gives it back but in the next shot he still has it.
(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.
The law enforcement officers that work for the Woodsboro Police Department are designated as Deputies. If it's called the Woodsboro Police Department, they should be designated as Police officers. If they're called Deputies, it should be called the Woodsboro Sheriffs Department.
(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.
The film takes place in California yet police vehicles are seen without a solid red light facing forward. According to the California Vehicle Code, all emergency and law enforcement vehicles in California are required to display at least one solid red light facing forward whenever their emergency lights are activated.
When Ghostface corrects Casey on the Friday the 13th question, he states that Jason didn't show up until the sequel. This is incorrect because in the original Friday the 13th, Jason makes an appearance at the very end of the movie, popping out of the lake and grabbing onto the boat.
(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. Suffering mental trauma can't really be called a 'motive' in any sense of the word.
(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.
Generally, for the fact there has been a massacre, the house in which it occurred seems quiet and serene, especially when Sidney discovers Tatum's body, but as if no one else would be around, or could have seen her. Sidney discovers Tatum's body rather late, when most party-goers have already left.
(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. The body on top of the van slides onto the windshield not because of Gale's sudden stop but because she went quickly into reverse.
(at around 28 mins) Obvious stunt double when Sidney is attacked for the first time and knocked to the floor.
(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!" (This is Stu speaking to himself from behind the mask, as it's clearly Matthew Lillard's voice. This also happens when Ghostface attacks Sidney in the police car. Stu can be heard yelling. This has been added deliberately.)
(at around 1h 40 mins) Stu's stunt double is visible when he charges Sidney and gets flipped over her near the end of the movie.
When Neve Campbell drops the TV on Mathew Lillard's head, none of the input cables are plugged in but clearly there are VHS tapes that drop with the TV.
It is announced @ around 50 minutes into the film that a city wide curfew will come into effect at 9pm. However, at around 58 minutes when the shops are seen to be closing at 9pm due to the curfew it is still broad daylight. Even if the events took place on the longest day of the year it would still either be pitched black or twilight at 9pm in California where the film is set.
While Casey is talking to the killer on the phone, there are successive shots of the unlit patio. Just before she is told to turn on the patio light ("again"), there is a shot of a figure (Steve) sitting on the chair. But in the preceding shot that shows the unlit patio, there is a shot of the chair and it is clearly empty. There was not enough time to get Steve seated and bound to the chair in only a couple of seconds.
(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 1h 14 mins) When Gale Weathers leaves the cameraman in the back of the truck she is heard to say "I'll be right back", but her lips say something like "keep watching".
Houses with multiple phones all ring simultaneously when a phone call is received, however, only the cordless phone from the kitchen rings after the killer's first two phone calls.
(at around 1h 8 mins) When Billy shows up at the party and goes up to see Sidney, Stu and Randy talk. At one point Randy says "Oh really Alicia?!" but his mouth says something different.
(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 30 mins) When Billy says, "...and if you get it right, you die," Stu bumps into the camera making it shake.
(at around 4 mins) When Casey is running to lock her front door, you can see the shadow of some equipment on the wall as she is running around the corner.
(at around 31 mins) Lighting equipment is visible when Billy is being put into the cop car.
(at around 39 mins) The camera crew is briefly visible in the reflection of the television after Dewey turns it off at breakfast, the morning after Sidney stayed at Tatum and Dewey's house.
Shadow in the school bathroom.
(at around 58 mins) The police station is supposedly on a one-way street, but a sign reading "Slow School X-ing" indicates traffic travels in both directions.
There should be a sensor on the garage door that would prevent the killer from stopping and closing the door as Tatum as attempting to exit.
Given how quickly Casey turns the lights back on, it's implausible that the killer would be able to gut Steve and move out of view in time.
(at around 1h 12 mins) At Stu's party, everyone watches Halloween - La notte delle streghe (1978). 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." Its still safe to say this is still a character error even if the characters are intoxicated. Drinking doesn't cause hallucinations and none of the party goers are shown using hard drugs that would explain them seeing blood when there isn't any.
Gale should know better than to try to doorstop Sidney after being attacked in her home. Sidney is a minor (the script says she's 17) and as a result cannot be legally identified on camera if she's associated with a crime. While Gale could have gotten an interview and edited it to protect Sidney's identity, naming her on live TV as one of the suspects is a massive oversight on her part.
(at around 1h 15 mins) Randy says "Here comes the obligatory tit shot" while watching Halloween - La notte delle streghe (1978) 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).
Stu simply planted the voice scrambler and cellphone into Neil's shirt without wiping his fingerprints or using gloves. Something the police would figure out and still be able to prosecute him and Stu for the murders foiling their entire plan.
In Sidney's bedroom near the beginning of the movie she asks her father "can you knock?" after he opens her door. He in fact does knock she just didn't acknowledge it.