Factual error: At the climax of the movie, Alice swings the machete at Mrs. Voorhees and decapitates her. When the blade makes contact with Mrs. Voorhees' neck, her head flies off at an incorrect angle, as though the cut began on the opposite side.
Continuity mistake: While breaking down the bathroom door with the axe, Jack repeatedly strikes and damages the right hand panel, but as he turns away from the door (when he hears the snow-cat) both left and right panels have been damaged.
Plot hole: After his mother's death, Peter sets a camera up outside her therapist's office, so he can see who goes in and who comes out; he thinks his mother's killer might be one of the doctor's patients - a tall woman with long blonde hair. All he sees is people going in and people coming out, and then he sees her - the tall blonde. Problem: He never saw her going in, only going out, which is critical, because the killer is the therapist - a tall man - wearing a long blonde wig and make-up. Of course no-one says anything about "We never saw her going in, only out, so it must be the therapist!"
Continuity mistake: When the windows on Nick's truck are smashed, Elizabeth has protective gloves on when she covers her face from the glass, they are then gone when the truck comes to a halt.
Continuity mistake: When the lady anthropologist gets out her teacher's car - you can see in the background the traffic lights changing from green, to amber, to red. When it cuts to another view of her - the traffic lights are on green.
Revealing mistake: During the scene in which Lou forces himself on Kim in the lunchroom, donning a black ski mask, Kim's brother Alex jumps up to his sister's defense. Just before Alex throws his first punch, Kim screams "Michael," referring to the actor's real name Michael Tough. Kim then correctly screams "Alex," the name of the character he was playing in the film.
Continuity mistake: At the end, the buried biker digs himself out of his grave and shuffles to the smokehouse. When he attacks Farmer Vincent, all the soil and mud stains have vanished from his clothing. Guess he must have stopped off at Zombies-R-Us for some fresh new duds. (01:30:00)
Revealing mistake: The cannibals kill Faye, and then decapitate her. Then one of the natives lifts her head up, only to let the wig slide off of the fake head, and reveal a yellow foam head underneath.
Visible crew/equipment: When John is playing the piano, after the door opens by itself in the music room, there is a shot that revolves around him. If you look in the large mirror in front of the piano you can see the pole of the boom microphone reflected (behind the chandelier). (00:19:21)
Visible crew/equipment: In the opening shot of the movie, several people (presumably the film crew) are reflected in the eye of the titular reptile. (00:00:15)
Visible crew/equipment: When the survivors of the cruise liner sinking approach the ominous ship, it's supposed to be deserted, but as a window opens, a person's arm holding a stick to open the window is visible.
Continuity mistake: Kenny must be fast. He's disguised as the female assistant at one point then minutes later he's disguised in the Groucho Marx costume.
Suggested correction: It's not a full costume. Kenny steals the mask and what looks like a small waistcoat (leaving even the hat). It's not implausible that Kenny made a quick change (probably after killing Ed) to formal wear (which isn't what Ed was wearing). After Ed's murder, Kenny-as-Ed appears briefly only from the waist up (showing only the stolen mask and waistcoat), and then doesn't appear again for quite a few minutes, more than enough time to change to the formal wear.
Revealing mistake: At the end of the film, it's obvious the ground that Phil falls into is just some mats dressed up in fake snow.