(at around 20 mins) The police chief conducts a press conference. Later, when videotape of the press conference is reviewed by Gale (at around 1h 17 mins), the person to his left (frame right) is different.
(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.
(at around 16 mins) Randy corrects Film Class Guy #1's quotation of Ripley's famous exclamation in Aliens ("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, 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 here, you bitch."
(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 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 25 mins) When Sid smacks Gale Weathers in the courtyard after being introduced to Cotton for the first time, Gale's hair alternates between being tucked behind her ears and being in her face in between shots.
(at around 57 mins) When Sid is practicing in the theatre as Cassandra, she lowers her veil around her head and overlapping in front of her as she says, "Save to speak the truth." When the shot goes wide she is still lowering her arms. She is also looking straight ahead in the closeup yet her head is tilted left and downward in the far shot.
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.
(at around 1h 24 mins) When Sidney and Hallie get in the police escort car Hallie gets in first sitting on the left, but later when the car is driving down the street and eventually crashes, Sidney and Hallie have switched places and Sidney is now on the left. (This was explained in an outtake. As a joke, the girls got out and ran around the car when it stopped at a red light.)
(at around 1h 21 mins) When the killer is stabbing Dewey and Gale is staring at him from the other side of the glass room, the killer's reflection is seen just staring back at Gale, when it should be moving around while stabbing Dewey.
(at around 34 mins) When the killer is after CiCi on the balcony, the killer stabs her in the back twice but when the killer removes the knife from her back both times you can clearly see there is no blood on it.
(at around 10 mins) During the last stabbing of Maureen Evans, the killer puts his right arm around her and stabs her with his left, but from the back view, his left arm is around her and he is stabbing her with his right hand.
(at around 1h 50 mins) At the end of the movie when Sidney shoots Debbie Salt/Billy Loomis's mother after she's already dead, the actress's face grimaces as Sidney shoots her though she is already dead.