The exterior and layout of the mansion changes from the first time the group of youngsters come upon it and then later when they are trapped inside. It is clearly two very different buildings.
When Bob throws an axe at Ronald, he misses and the axe lands on the floor. Yet, the axe is miraculously back in Bob's hand with the following cut in the scene.
When the two last survivors are sitting on the floor after defeating the girl, a shadow of a wire can be seen behind the survivor on the left.
When the decapitated head of Abigail is on the floor and screaming, you can see the floorboard moving up and down, clearly showing that they are in fact not wooden floorboards at all.
Joey never saw the old woman, but when he is hanging out of a window, when he sees her, he immediately pleads for help, while in reality he would probably be scared because he wouldn't know who she is and how she got there in the first place.