The opening scene was originally supposed to take place in the basement of the house. Through a misunderstanding, though, it was locked up and unusable. The scene had to quickly be changed to fit the room it ended up being shot in.
The brace the killer used on his leg made it very hard to get around. Multiple times the actor fell down due to the heavy cast covering his leg.
The scene with "The Couple" was originally filmed with different actors. When one of the originals couldn't finish the rest of their scenes, it was recast with two new actors. The biggest change here is that it switched from two girls to a guy and a girl.