Goofs

When Melissa checks on Lauren before Mrs. Mac and Heather are about to leave, you see that Heather's bag is on her bed (Lauren and Heather are roommates), but when Kelli and Leigh rush in and Melissa is sitting down, the bag is gone.

System alert on Megan's laptop during the blackout and the video stop frame it covers change their sizes once Kelly notices them.

There is a scene in which Constance is trying to get pregnant, but is disappointed by her lover's repeat failure to perform. She decides to use 12 year old Billy instead to get her pregnant. As she leaves her lover on the stairs and approaches Billy in the attic, Billy is continually rocking in his rocking chair. It is clearly intended that the rocking motion is continuous, but the angle between the chair's rockers and the floor boards varies depending on the view of the camera. When the camera is pointed back towards the attic door, the front of the rockers is angled about 20 degrees to the left of the floor boards. When the camera is showing various objects on the floor like pots and pans, the angle changes to more like 45 degrees to the left of the floor boards. Finally, when Constance stands nude in front of Billy to offer nookie, the angle changes again, this time to about 20 degrees to the right of the floor boards.

When the girl is trapped behind the wall, you can see Billy coming from below, he then grabs another girl from behind the wall on an upper floor, but then he's clearly shown still coming from below.

It is stated numerous times that the sisters would have a hard time escaping the house as it is hours away from safety. As per rules, all college sorority houses are situated on a college campus therefore they should have been able to reach safety.

The bathroom floor is shown as paper-thin as the walls, with Billy making multiple holes in it to watch the girls showering, and there's no way it would've been able to hold people's weight on a regular basis.

When Billy bashes his mother with a rolling pin, one of the handles breaks down and loudly flies away. However, when Billy throws a rolling pin on a table, it still has both handles intact.

You can not see anyone's breath when they go outside, even though it is supposed to be really cold.

When Billy stabs his stepfather in the eye, the blade comes out of the back of his head with the eye facing pupil-out, although that could not happen if the blade went through his eye.

Michelle Trachtenberg's character has her name spelled in the credits (M-e-l-i-s-s-a) differently from the way it appears on screen as part of a Caller ID (M-e-l-l-i-s-a).

Dana's eye noticeably twitches in the crawl space scene, even though her character is supposed to be dead.

After Kelly breaks the window in her hospital room to escape. You can see a nurse in the hallway behind her, who could have helped. When the camera turns back, no nurse. Also, when she knocks down the nurse cart when running away, not even those nurses come to help her out.

The Canadian short-print blu-ray contains a typo, the back cover states that there are only 2 alternate endings available. However, when playing the special feature it can be seen that all 3 existing endings are available.

When Ms. Mac is first telling the story of Billy, she eats a marshmallow that she just cooked in the fire. There is no way that the marshmallow and the tongs that just came out of the fire place wouldn't have burned her mouth.

The heavy snowfall makes it more difficult for the girls to leave the sorority house, so then how did Billy and Agnes even reach the house in the first place?

When these girls are being killed they constantly scream bloody murder yet for some reason no one can hear them.

Why are Billy and Agnes cannibalistic? They eat and consume the flesh of some of their victims completely randomly with no explanation.

Melissa explains to Heather that Billy is not a serial killer because he didn't murder for sexual thrill and is instead a spree killer because he lost his mind. She's not entirely correct, the only difference between a spree killer and a serial killer is the lack of a "cool-off" time. A spree killer is someone who kills multiple people at one time or in quick succession while a serial killer is someone who kills multiple people in distinct incidents. Although Melissa is right that Billy is a spree killer, she doesn't have the correct reasoning.