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.
Continuity mistake: When Drew goes to answer the door to the trick-or-treaters, the shot of the table at the front door has four legs at the corners. When the close up shot of it is taken, it's now a totally different table with a single middle post.
Visible crew/equipment: When Cindy and Buffy are lying on their own beds, and then Doofy comes in, a boom mic is reflected in the mirror behind Buffy, slightly moving backwards when Doofy opens the door. (00:41:14)
Revealing mistake: When Bobby takes Cindy to her room after she drank out of the beer bong, it's all blue outside the window.
Continuity mistake: When Cindy is fighting the killer during the Matrix spoof, there is a short, storage table to the right of the plaid curtains. When the killer flies backwards through the window, that table is to the left of the curtains. (01:14:00)
Continuity mistake: During the final confrontation scene between Cindy, Bobby and Ray, watch the knife that Ray holds. During close-ups it's a hunting knife with a fairly large blade. However, during a couple of the long shots, the knife blade is a different style and much shorter, obviously the prop knife used for the stabbings.
Continuity mistake: In the scene when Cindy comes out of the school where the jumper is on the roof, its very sunny just a few feet beyond the door. However in the shots where Cindy is facing the school and when she screams "What are you waiting for" - the area is now completely shaded.
Visible crew/equipment: At the start, just before Drew grabs the banana, a boom mic is in view in the window behind her.
Continuity mistake: During the "I Know What You Did Last Summer" sequence, after they swerve and hit the fisherman, their car is directly on the lines, but in the next shot it's not on the lines, then they are again.
Continuity mistake: When Cindy is walking away from the bum after giving him a sandwich, we see a white VW van driving away over her left shoulder. When the bum throws the sandwich back at her the van is parked back by the sign again.
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.