Revealing mistake: When Frank ties the bedsheets together to the sink, he ends up pulling the basin off the pedestal. Surely that would cause a flood, but we don't see any water come out at all.
Continuity mistake: When the wardrobe falls onto its side you can see that Frank hasn't broken the base. However, when he hides in it out on the landing, he is able to walk about inside it, moving the wardrobe with him.
Continuity mistake: When the other guest goes out into reception and asks "Mr. Bedford, all quiet?" in one shot he is wearing his dressing gown, but the next he's just wearing pajamas.
Continuity mistake: When Frank and Betty place their wardrobe down, it nearly goes through the floor because the boards are damp. First Frank tries putting wood under it (from the drawers), then after breaking the wardrobe door he swaps the wardrobe with another guest's. They place that wardrobe in the place where the damp boards were without putting any wood underneath, and don't seem to have any problems with the damp floor.
Revealing mistake: When Frank and Betty move the wardrobe, the walls wobble.
Other mistake: As Frank and Betty move the wardrobe a chair tips over by itself. Frank and Betty are nowhere near it - it is presumably attached to a wire which is pulled by one of the crew, but has no significance to the plot.
Revealing mistake: Before Frank pulls the mat from under the wardrobe and tears it, you can see the mat is already torn and placed back together.
Audio problem: After Frank has torn the lino by moving a bed, he tries to neaten it up only to tear it more. The sound effect is not in line with the tearing of the lino.