When Kirsty opens the box while in the hospital, the walls begin to part and you see tons of cobwebs being stretched between the two walls. In the next shot, Kirsty is shown walking toward the opening but the cobwebs are gone.
When Kirsty discovers frank he delivers his catch phrase "Come to Daddy." Later, when Kirstyis fleeing the house with the puzzle box the line is heard again but it is in Larry's voice even though Frank hasn't replaced him yet.
When Frank first opens the puzzle box in the beginning of the film, the inner sides of the sliding pieces are white. Later, when Kirsty opens the box in the hospital, the box's inner sides are brown woodgrain, just like the exterior of the box.
When Kirsty first wakes up in the hospital bed, the tray-table in front of her is empty. The doctor comes in and deposits the puzzle box on the table. He says he'll get her a phone after they've talked. However, directly after he leaves, we see that a phone has appeared on the table. It is neither mentioned nor used.
At the start of the film where Frank is buying the box, when the camera is zoomed in on the box, you see two glasses, but when the camera is zoomed out there is only one on the table. Also throughout these shots, everything on the table moves around.
At the start of the film when Kirsty is walking towards the house you can see her hair is evenly sitting behind her head, but in the next scene when the camera looks back at her, her hair is bunched to one side.
When Kirsty bumps into the corpse and the maggots fall out,that would suggest that the decomposing dead man had been there for some time. Even if it was well hidden by Julia(and perhaps also by Frank) the smell of a decaying body or two would be noticed.
When The Monster in the tunnel chases Kirsty back towards the hospital room, you can see the trolley and wheels the monster is being wheeled on. Also whilst Kirsty is running away, the camera cuts to the tunnel, and it seems she is running the wrong way (the tunnel with the painted disappearing point appears supposedly ahead of her).
When seen from inside, the attic has a single window made from four separate panels of glass. However, from the exterior shots, the real house has six panels. It makes obvious that the internal scenes were shot elsewhere: in fact, they were the only scenes filmed in a sound stage.