(at around 1h 3 mins) When the killer talks to Randy on the phone, he mocks Randy about not being the leading man, to which Randy replies "Fuck You!" But when Gale and Dewey watch the killer's tape (at around 1h 18 mins), Randy says "Fuck You!" before the killer mocks him.
(at around 1h 18 mins) When Gale kisses Dewey in the lecture theatre, all her dark red lipstick is wiped off or smeared around her mouth, but none of it appears on Dewey.
(at around 1h 25 mins) Red drawings on Derek's chest disappear/reappear when the fraternity captures him.
(at around 5 mins) When Maureen Evans goes up to the concession counter to order her popcorn and Pepsi, the clerk gives it to her without accepting any money, or ringing it up on the till, yet we see her with the money in her hand.
(at around 10 mins) We see Maureen Evans being stabbed repeatedly by Ghostface, but when she turns to face the crowd at the theater the amount of blood in no way matches up with her wounds.
One of the killers stabs a knife through the bathroom wall and into Phil's head. Bathroom walls are too hard to stab a knife through, much less both through that and through a hard skull.
Blood comes out of Phil's mouth as a result of him being stabbed in the head, but there is nothing that should have made blood come out of his mouth.
Toward the end of the film Debbie Salt/Billy's Mother claims that she killed Randy. However this does not make sense. When Randy is on the phone with the killer right before his death, the killer mocks Randy. The killer references what Randy said to the students in his film class about "the geek getting the girl". Mickey was in the film class, & heard Randy say this. Billy's Mother was not in the film class, & did not hear him say this.
Even if she wasn't present, she and Mickey were in cahoots. So it's plausible he told her how she could get Randy's goat.
(at around 1h 26 mins) Blood on the younger cop's throat before it is slit.
CiCi is on the top balcony, CiCi gets stabbed by ghostface and then the knife is thrown away. CiCi is then thrown from the balcony. Once seeing the dead body ghostface wipes the blood from the knife that was thrown away.
In the finale the ropes around Derek are slightly loose and show no signs of holding his weight.
(at around 1h 6 mins) When Sidney gets the instant message from the killer, in the library, the man next to her tells her to hit Alt-M. She punches a lot more than two buttons to bring up her instant message.
As Cici is being thrown off the balcony of the Omega Beta Zeta house after being stabbed, you can see the head of a male crew member appear to the bottom right of screen.
(at around 1h 24 mins) When the cop car first pulls away, a reflector screen is visible in the frame of the back door.
1hr 45 min, when Sidney made the theater rocks to fall on Debbie Salt, when Debbie fell, she fell on a mattress. Mattresses are used in films to soften the actor's falls. This is clearly not meant to appear on film.Crew/equipment visible.
Reflected in most of the vans' windows.
There is a Los Angeles County Seal on the rear window of the ambulance at the end of the movie. Though the location of Windsor College is not explicitly stated in the film, one can infer that it is meant to be located in southern Ohio, as evidenced by the police cars using Ohio license plates, the local news reporter's microphone at the beginning features the words "Ohio 66," and the area code on the caller ID of Sidney's prank caller in her opening scene displays the area code 513, which is reserved for the southeastern corner of Ohio.
The fact that Sidney comes within close vicinity of Debbie Salt/Mrs. Loomis a few times in the movie and doesn't recognize her is absolutely insane. Even with a large college campus, the audience can see them almost cross paths several times in the movie. Sidney would have easily noticed a lot sooner who Debbie Salt really was.
(at around 16 mins) Randy corrects Film Class Guy #1's quotation of Ripley's famous exclamation in Aliens (1986) ("Get away from her, you bitch!"), but Randy is wrong; Film Class Guy #1 was correct. Film Class Guy #1 was supposed to misquote Aliens (1986), but Joshua Jackson spoke Jamie Kennedy's line. Thinking fast, and to keep the scene going, Kennedy spoke the quote intended for Film Class Guy #1, "Stay away from her, you bitch."
(at around 36 mins) One of the running themes is characters discussing sequels that were better than the originals. At the Lambda party, Mickey mentions Star Wars: Episode V - The Empire Strikes Back (1980). Randy replies that it was not a sequel, but part of a trilogy and was completely planned. This is not true. We have learned from the documentary From 'Star Wars' to 'Jedi': The Making of a Saga (1983) that George Lucas originally intended Star Wars: Episode IV - A New Hope (1977) to be a standalone movie, and only wrote the sequels after the popularity of the original.
When Gayle says "There were more victims before the home stretch" and then names Tatum and her camera man, but they were the only victims killed at the final party; the murderers were the only other ones to die. Principal Himbry was the only victim between the first three and the "home stretch".
Towards the start of the film when the reporters are trying to interview Sidney one of the reporters can be heard calling her 'Sindy.'