Crew or equipment visible: When the Engineer chases Kirsty, the trolley and crew pushing it are visible.
Continuity: Throughout most of the film, Larry has a bandage on his right hand. When he talks to Julia through the bathroom door, Larry's bandage is on his left hand.
Continuity: 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.
Continuity: The amount of blood on Julia from inside the attic to the hallway to the bathroom.
Continuity: 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.
Continuity: The number of cups on the table during the opening scene (where Frank buys the box) changes from one to two then back to one.
Revealing mistakes: 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).