(at around 18 mins) In the flashback, when Shorty is pulling up (car still moving, but nearing a stop) the inside of the car is clear (no smoke). As the car stops, the inside is completely filled with smoke.
(at around 37 mins) When the piano falls down the staircase, it is straight, but later we see after the killer escapes from her house and Doofy is at the front door, behind Cindy the staircase is spiral.
(at around 45 mins) When Miss Mann gives the safety tips, an African-American girl stands behind her. That girl puts an article of clothing on, but she stops and leaves it curled up around her neck. The shot changes to the girls, then back to Miss Mann. This takes about four seconds. When it gets back to Miss Mann, the girl in the background is fully clothed and is standing relaxed and listening to the speech.
(at around 12 mins) The report card that Greg holds while cursing is different from the report card that he shows to Buffy.
(at around 1h 14 mins) Cindy throws a giant brass plate at the killer during The Matrix (1999) parody. When the scene cuts to the killer with a back injury, there is no sight of the plate.
(at around 57 mins) Shakespeare in Love (1998) jumps about half an hour and then jumps back about fifteen minutes in between shots in the movie theatre scene.
(Around 44min 50sec) When the sheriff is doing a press conference Gail Hailstorm is raising her hand with all the others and he says "Yes, Mrs. Thunderstorm" and she answers to it.
(at around 43 mins) The same girl walks by two or three times when Cindy and Bobby are talking in the cafeteria. Shorty even refers to this, making it clear to most viewers that it's a joke.
(at around 26 mins) Ray's jersey number on the field is 13; later in his room (at around 51 mins) it's 12. However, some high schools use different numbers on practice and game jerseys.
(at around 12 mins) Ray asks Greg if his yellow shirt makes him look gay. Then the camera cuts away; when it cuts back, Ray is wearing a black t-shirt instead of the yellow one he just had on because he put it on over the yellow one.
(at around 19 mins) When they dump the body into the harbor, they take him out of the trunk set him down on the wharf. You can see the fisherman set his hand down to catch himself.
(at around 36 mins) Wire visible on piano rolling down stairs.
During Shorty's snatch and run when the cashier behind the counter pulls his pistol and starts shooting, he isn't squeezing the trigger despite shooting noises happening.
(at around 33 mins) While Cindy is taking a bath she shaves and the razor is drawn along her wet hair but does no damage to the hair, revealing there are no blades.
(at around 1h 7 mins) When Cindy and Bobby are having sex, the pillows on her bed disappear and reappear several times.
As Shorty finishes his freestyle rap, Dave Sheridan goes for a high-five and Marlon Wayans leaves him hanging, so Dave swats Marlon's chest and then Marlon gives a return high-five as Dave starts the Killer's freestyle.
When Cindy and the Sheriff realize Doofy is the killer, he runs out of his office and ask two officers if they've seen Doofy. When they reply they haven't, he then asks everybody if someone saw him. One cop says that Doofy ran outside but, no cops are seen moving their mouth when this was said.
(at around 31 mins) The judge's mouth does not move when saying "must be an original piece" about Buffy's dramatic reading in the beauty pageant.
A crew member is visible through window of Cindy's room during one of the first scenes when we are introduced to Bobby.
When the garage victim becomes trapped in the 'doggie door' within the garage door, kicking her feet in terror, a hand can be seen pushing off her left shoe.
When the sheriff is doing a press conference in front of the school (49:23 minutes into the movie) he is calling reporters names to ask questions and he calls the reporter Mrs. Thunderstorm when her name in the movie is Gail Hailstorm.