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)
Continuity mistake: Near the beginning Buffy tears off her dress and throws it on the ground. When they walk away there's no sign of it. (00:09:20)
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)
Continuity mistake: In the scene where Buffy and Cindy get dressed in the locker room, a girl standing right next to Buffy first drops her towel to get dressed and a few seconds later has the towel wrapped around again. (00:44:05)
Continuity mistake: In the scene where the killer is in Cindy's house, the killer is hiding behind the couch. When you see shots of the killer, the cell phone is in his right hand and the hook is in his left. When she notices him and he stands up, the phone is in his left hand and the hook is in his right. (00:33:30)
Continuity mistake: When the killer jumps up behind Ray and stabs him through the back with the hook, we see he obviously gets lots of blood on it. Watch it when he goes to stab Cindy and misses. It hits the cupboard and is completely clean. The hook is clean for the rest of the time he has it. (01:10:50)
Continuity mistake: At the end of the Matrix scene he throws Cindy into the wall, look when she hits the wall there is no sign of hitting the wall, but when she is falling to the ground they is an imprint like an oval on the wall. (01:11:05)
Continuity mistake: When the six characters are imitating the scene of I Know What You Did Last Summer, they get out of the car after it has spun around. All of the headlights are on, the two yellow and the two white. Later, if you look behind them, the left headlight is off. (00:15:45)
Continuity mistake: In the scene where Ray is killed, the killer stabs Ray with his hook. Ray's shirt is not bloody, but as he falls it is. Then when he lands on top of Bobby his shirt once more is clean. (01:10:50)
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)
Visible crew/equipment: When the teens are all standing out side and the teacher is ready to jump from the school building. As he is jumping you can see the shadow of the scaffold he jumped from in the left bottom hand corner of the screen. (00:48:20)
Continuity mistake: In the flashback of the gang killing the fisherman, Shortie drives up in his car and just as someone says, "Brenda take care of them" there is a shot of the car with no smoke inside it. The camera cuts to a view of Brenda approaching and the car is full of smoke. (00:17:30)
Continuity mistake: When the bottle is dropped in the car, it hits Cindy on the head and falls down in front of her. When Greg gets out he now has the bottle. (00:15:15)
Continuity mistake: When Greg stands up through the car's sun-roof, during the flashback, he is holding an empty bottle, but when he is at the top of the car the bottle is nearly full. (00:15:20)
Continuity mistake: In the scene after they look at Greg's report card, Ray has his hand on Brenda's ass, in the next angle it is on her shoulder. (00:12:05)
Continuity mistake: In the scene where they hit the fisherman, they spin the car. Notice the yellow line, they are somewhat over it in one scene, then in the next the line in to the right of the car, not under it. (00:15:55)
Other mistake: At the start just after Drew is seen in the line of the sprinklers, the killer is seen walking towards her with his knife held high in his right hand. If you look closely in this shot you can see blood on the knife despite not stabbing Drew yet. In the next shot it is gone. (00:03:45)
Continuity mistake: When the gang hit the guy with the car they all get out. When Greg gets out he only has the bottle in his hand but when the camera returns he is holding a flashlight. (00:16:00)
Visible crew/equipment: After the garage door comes down on the garage victim, a hand can be seen from the outside pushing her boot off her foot as she's frantically kicking her feet.
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.