Audio problem: When Gale is talking to Kenny the cameraman before she goes off to investigate the abandoned car with Dewey her face is in a close up. She can be clearly heard uttering the forbidden phrase "I'll be right back." However her mouth can clearly be seen saying "keep watching."
Revealing mistake: When the killer surprises Sid after she's come in from outside and locked the door, he tries to stab her with his left hand, misses, and hits the door right next to the chain in closeup. In wide shot (very quick shot) when Sid legs it, look at the killer and you can see he's holding a very badly bent knife in his right hand, nowhere near the chain.
Continuity mistake: At the very end, when Sid finds out who the killers are, they are all standing in the kitchen. When Billy and Stu stab each other take a look at the blood stain on the left side of Stu's jumper. It starts around his shoulder and vertically goes down towards his waist. When Sid is fighting him in the lounge she gets up and goes behind the TV. In the shot looking straight down on Stu from a birds eye view, notice the blood stain is now horizontally across his stomach. You'll have to look closely for this one.
Continuity mistake: Pay attention to the scene where Randy gets the call that the principal is dead and the rest of the group races off to try to see the body before they cut it down. Five or so people get up and run towards the door. When they are shown in the exterior shot running from the house to the car it is an entirely different group that are not even dressed similarly. This was due to the fact that the group were made up of actors whose contracts were by the week. The exterior shot was the only thing they didn't get during that initial week and to bring them back for that one shot they would have had to have been paid for an entire new week, which was too expensive for one shot. They just used extras instead.
Revealing mistake: After Gale backs the van up, and Kenny's body falls on the windshield, they cut to a shot where Gale is driving away, and Kenny's body is on the roof of the van. When she starts driving you can clearly see Kenny lift his head into the air to look forward, even though he's dead.
Continuity mistake: When Casey is making popcorn, firstly the top of the popcorn that she tears off and places next to the stove changes from being folded in half, to later on laying down flat. Secondly Casey picks up a salt shaker and places it by the stove, but this later disappears.
Continuity mistake: At the beginning of the film, Casey is cooking popcorn on the stove. When she peels the top off, you see her peel the side away from the handle, and she's using her left hand, but when the shot changes, she's using her right hand, and is pulling from the other side.