Revealing mistake: During the scene when Cindy and the killer are fighting Matrix-style, Cindy kicks the killer, and he knocks down the book case so it falls over him. There is a vase with flowers on the book case - when it falls down the vase still remains on the book case, glued to it. (01:11:30)
Revealing mistake: During the final party scene, where Cindy is wearing a Viking Hat and yells with joy as she drinks from a "Beer Bong", if you look closely, right before they pan to Cindy, you can see that the hose of the "Beer Bong" is not attached to the funnel, or anything for that matter...it is just held alongside it. (00:58:40)
Revealing mistake: In the snatch and run scene right at the end of the film, when the store owner gets his gun, you can see he doesn't fire it, there are just sound effects. You can easily notice this because the owner's hand doesn't move when he "fires".
Revealing mistake: At the very start of the film when Cindy hears a noise outside her window and goes to check, she was on her computer. But if you look closely when she stands up you can see that the computer isn't even switched on. (00:04:35)
Revealing mistake: When Cindy's dad gets his head stuck in the doorway at the start, to see if Cindy is okay, his head is not stuck as you can see a small space to the side of his face. It's not a lot, but enough to not get stuck. (00:04:55)
Revealing mistake: When Cindy is dancing in the air after kicking the killer, she is very obviously a CGI. (01:12:10)
Revealing mistake: When Cindy is in the bath and she starts to shave her face, she repeatedly shaves the long head hair which has fallen over her face, and yet it is obvious none of it comes away with the razor, indicating there isn't a blade in it at all. (00:32:15)
Revealing mistake: When the teacher jumps it's clear the ground is a mattress with grass and leaves over it. Soil would not bend the amount it does, even with someone jumping off the roof of a building. (00:48:20)
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)
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)
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.
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)
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)
Revealing mistake: At the end in the "Matrix" fight scene, the killer throws Cindy against the wall, which moves at the top. Watch the gap at the top of the wall. (01:11:00)
Revealing mistake: In the scene where Greg and everyone are doing the "I Know What You Did Last Summer" parody, Greg has a Jack Daniels bottle. After Bobby turns on the music Greg pushes himself up to the sun-roof. If you watch carefully the bottle bends as he pushes himself up.
Revealing mistake: When Bobby takes Cindy to her room after she drank out of the beer bong, it's all blue outside the window.
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.