Deliberate mistake: When "Legs" lowers the Pez duck named Ducky down through the removed lightbulb hole, to ring the doorbell, the lightbulb housing is bigger than the hole but they bring it up through the hole to remove it. (01:03:55)
Deliberate mistake: After James and Kat enter Whipstaff and get the power turned on, when Kat heads to the staircase to go search for a bedroom, we can see a few of the metal stair runner rods attached to the stairs to keep the carpet runner in place. However, when James falls at the top of that staircase and gets rolled up in the carpet runner like a sushi, there are zero stair runner rods. (00:21:30 - 00:31:50)
Deliberate mistake: When the house is splitting in two, the board game should have fallen long before it actually does. (01:25:25)
Deliberate mistake: The surviving passengers are all awoken from a deep sleep after Dinah realises the other passengers have gone. Yet none of them appear drowsy or tired and none of their eyes appear strained, despite having just woken up so suddenly. (00:09:20)