Continuity mistake: When Cindy is running up the stairs and throwing the bicycle and piano at the killer the staircase is straight but when she goes to open the door and sees Doofy, if you look behind her the stairs are spiral. (00:34:30)
Revealing mistake: When the killer is getting kicked into the glass cabinet you can see a string pulling the glass so it will break. (01:14:25)
Continuity mistake: In the scene in the bedroom between Cindy and Bobby, just before he explains about sucking on a tootsie pop, Cindy's hair is all messed up. Camera cuts and it is fairly different. (01:02:15)
Revealing mistake: When Cindy is thrown into the wall just before the Matrix style fight, the wall bends, showing it is made of rubber or foam to protect Anna Faris. (01:11:05)
Continuity mistake: When Ray tells Brenda to put on his football gear, he hands Brenda his shoulder pads. When Brenda accepts these she has two braids hanging down in front of her. The camera cuts to Ray for a second then back to Brenda. When the camera has cut back the two braids have moved over her shoulder. She couldn't have moved them with her hands as she is holding Ray's shoulder pads with both hands. (00:49:30)
Revealing mistake: When Cindy walks in mid-air Matrix-style, she knocks the killer backwards. In the next close-up, as he falls on his back, the outline of the thick protective padding worn under his clothes is visible.
Continuity mistake: The living-room in Cindy's house has wood plank flooring, which is perpendicular to the plank flooring in the hallway, as seen at the doorway when she runs down the stairs and locks the front door. However, when she runs up the stairs as the killer chases her, the living-room wood flooring at that doorway is gone.
Deliberate mistake: When Cindy is talking to the killer on the phone, and he says, "I could be anywhere. I'm like the wind, baby," she walks into the living-room. Notice the wall behind the couch, it is completely different a moment later. When the killer hides behind the curtains, the wall behind the couch has large bay windows with seating underneath. Then when the killer is at her house the second time, all the windows are gone during the Matrix send-up, so Cindy can run along that wall.
Revealing mistake: When the killer throws Cindy against the wall, causing the painting to fall, the outlines of the knee pads she's wearing under her pants are visible, while she's in mid-air. (01:14:20)
Visible crew/equipment: During the Matrix send-up, after Cindy throws the platter at the killer, an actor's T-mark is visible on the floor, when he leans back and flounders about.
Continuity mistake: When Cindy and Bobby are talking in the school cafeteria after he gets out from jail, the students in the background keep disappearing as they go off camera from shot to shot when they should be seen walking away behind the characters.
Continuity mistake: When Bobby and Cindy slowly begin to make-out on her bed there is a TV set behind them, but after Bobby gets his finger shocked on Cindy's "underwear" and the both of them get up to leave, the TV set is gone.
Continuity mistake: When Ray is with Brenda in bed, and she get out, we can see her pillow in one form, and in the next scene is in another form. (00:49:20)
Audio problem: The judge's mouth doesn't move when he says, "Must be an original piece" about Buffy's dramatic reading at the pageant.
Continuity mistake: Just after Brenda drops the first beer bottle, her hands are out in the "don't shoot" position. The camera cuts and she is holding the bottle in both hands. (00:57:50)
Audio problem: When Brenda is in the movie theater, people are hurting her and yelling at her, but no one's mouth moves. (00:56:30)
Continuity mistake: In the scene where Cindy was fighting the killer Matrix style, she threw a brass ornament at the killer like a Frisbee and the killer dodged it, and the brass ornament should have gone out of the window by smashing through it. But the window was broken by the killer flying out of the window after Cindy kicked him/her. The brass ornament has mysteriously disappeared into thin air. (01:11:40)
Continuity mistake: After they dump the body in the water, Greg tries to get Cindy to agree to the pact. When she doesn't he starts strangling her. When he lets go, Cindy starts to reach her hands to her neck but in the very next shot her hands are at her sides. (00:19:35)
Audio problem: In the scene in the teacher's classroom, when the teacher says "I'll see you after class". Two guys say "Sweet" and "Alright" and high five each other. However, their mouths don't move at all. (00:23:10)
Revealing mistake: When the man in a striped shirt stabs Brenda at the movie theatre, he pulls the knife out and there is no blood on it. (00:56:05)
Chosen answer: I know for sure they were making fun of the movie 'Usual Suspects' (dropping the cup) - they might have added the gag of the fish in it to appear as though she was shocked by the fish instead of the true identity of Doofy.