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."
One of the running themes is characters discussing sequels that were better than the originals. At the Lamda party, Mickey mentions Empire Strikes Back. 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 Star Wars to Jedi: the Making of a Saga that George Lucas originally intended Star Wars to be a standalone movie, and only wrote the sequels after the popularity of the original.
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.
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, Randy says "Fuck You!" before the killer mocks him.
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.
When Sidney is rehearsing in the play as Cassandra, after she gets up, in one shot she is covering her face with her veil and in the next shot, she has her arms and her veil up and she is lowering them.
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 Sydney's prank caller in her opening scene displays the area code 513, which is reserved for the southeastern corner of Ohio.
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.)
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.
In the scene where 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.
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.