Continuity mistake: At the start, a plate of cheese comes out from underneath a painting, which Wallace follows, causing him to hit the wall. In that shot, Wallace's bed is very close to the wall, and on the other side there is a white chart displaying different kinds of cheeses. But a few seconds later, as the bed in sent backwards, it's much further away from the wall. Also, the painting is much higher up than before, a machine has appeared underneath it, and the white chart has disappeared.
Revealing mistake: As Lady Tottington leaves Wallace's home, the ivy between the windows on the left suddenly changes a bit- a portion gets squashed flat (about 2 bricks down and to the left of the house address numbers) as the area gets a bit blurry. The modelers must have not noticed bumping it while working with the doll.
[Quartermaine's hairpiece has been sucked up in the bunvac.]
Lord Victor Quartermaine: I want...toupee.
Wallace: Oh, yes, of course. We take cheques or cash.
Lord Victor Quartermaine: No, you idiot. My hair is in there.
Wallace: Oh, no, only rabbits in there. I think you'll find the hare is a much larger creature.