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.
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.
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 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.
Billy's family really only consists of 4-5 members (his daughter/sister Agnes, his mom, his dad and stepfather) yet for some reason his former home consists of 9 rooms for the 8 sorority girls and the house mother.
When Mrs. Mac escorts/pulls Kyle down the stairs, an Exit sign (such as you might see at the ends of corridors in large buildings) is visible at the end of the upstairs hall, just past the stair landing. There would be no reason to have a commercial Exit sign in a sorority house/personal residence.
The goof items below may give away important plot points.
When Billy's father is killed and getting buried under the crawlspace, he is seen with the black bag over his head. When Dana is fighting in the crawlspace and is killed, she has scratched up part of his skeleton; however, there is no bag over his head.