The ring(s) on Chelsea's left hand change during the scene in which she is in the attic. This is particularly visible when she reaches down through the floorboards and is grabbed.
At the beginning when they are all driving to the castle in a terrain vehicle, the license plate is a Romanian number from Bucharest (B-0...) and when they reach the castle in the night the plate is an American one.
When Mike and the others are in the Hellworld party host's study, he's wearing a gray hooded sweatshirt with no jacket. When they go down the stairs into the lower parts of the house, he's suddenly wearing a leather jacket over the sweatshirt.