Revealing mistake: When Fool fall off of the edge of the bathtub it is quite obviously a taller stunt double used instead. (00:42:00)
Continuity mistake: When one of the People from Under the Stairs bursts through the wall, trying to attack the Woman, the hole it makes in the wall has wood splinters hanging all around it, yet in the next shot it is completely free of any splinters and looks much more neater. (01:28:45)
Continuity mistake: When Fool first arrives at the house, dressed as a bear trooper, the whole lawn is sunny and bright. In the next shot the house casts a huge shadow over the lawn, making it dark. (00:11:50)
Continuity mistake: When all of the People from Under the Stairs attack the Woman in the kitchen (as she tries to stab Alice) one of the people climbs out of one of the cabinets twice between shots. (01:29:55)
Factual error: When Fool calls the police from a coin phone, he puts the coin into the phone, then calls 911. Given that 911 is a free call we should hear the sound of the coin being returned immediately after it was dialed, but we don't. (01:08:45)
Continuity mistake: When Fool falls downstairs, the hatch in the door is open (and sunlight streams through it) but when Roach chases after him and Fool runs back up again, the hatch is now closed. (00:29:15)
Continuity mistake: When Fool and Leroy first encounter Prince, they secure him in the room beside the kitchen, with the door to the outside closed. Yet when Mummy and Daddy return, the dog somehow has managed to get out of the super-secure house to greet them. (00:20:10 - 00:30:30)
Plot hole: Why is it possible for the people under the stairs to bust through the walls and stairs but not through the wood slats that keeps them in downstairs? Especially after the boards have been weakened by gunshots. (01:32:00)
Revealing mistake: When Alice bursts out of the ceiling and knocks the Woman out, her kneepads are visible. (01:23:40)
Continuity mistake: The amount of blood on the side of the Man's face changes alot through the movie, especially in the cellar scenes. (00:50:20)
Factual error: When the cop (Bobby) asks the woman if it's her van, she says she found it when she got home. In actuality, the police would get suspicious because she didn't report it. Someone comes home and sees someone else's vehicle parked in their driveway and neither her or the man call to report it (at the very least to have it towed)? (00:44:15)
Revealing mistake: When Alice trips and falls on the staircase, the Woman gets on top of her and tries to stab her. Throughout their struggle, you can clearly see that Alice is in fact replaced by a stuntman. (01:28:00)
Revealing mistake: After Alice stabs the mother with the knife, the mother begins to take it out. However, you can see that she's taking it out from the side of her body and you can even see the point of the knife on the other side. (01:29:05)
Continuity mistake: When Leroy barricades the metal door, her uses a mop. He puts the mop inside a drawer, and the mop head hangs out of the drawer. Later on when the Woman and Man come back, we can now see the mop head is tucked inside the drawer. (00:19:20 - 00:31:05)
Revealing mistake: When Alice is forced to sit inside the bathtub of scalding water, as the Woman pushes her inside it, it is quite obvious that the steam rising from the tub is actually coming out from under the tub. (00:50:50)
Audio problem: As Fool is attacked by Prince in the wall he cries out "Alice, Get Back!" yet his mouth is shut. (00:59:20)
Revealing mistake: When Prince attacks Leroy, in some shots it is quite apparent that Prince is just a stuffed animal. (00:32:25)
Factual error: When the Man cocks his gun to pursue Leroy and Fool, Leroy refers to the gun as an "automatic." The gun was actually a semi-automatic handgun. (00:33:30)
Continuity mistake: When Roach runs up the cellar stairs after Fool, and grabs his feet, we see one of Roach's fingernails fall off. In the next shot the nail has vanished from the floor and isn't seen. (00:29:45)
