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.
Continuity mistake: When Gromit presses the button to deploy the lasso, there is a hood ornament on the car, which rotates 90 degrees as the hood opens. But a moment later when the lasso rises (and for the remainder of the chase) the hood ornament is gone. When the car returns to Wallace's house and shakes off the mud, the ornament is back again.
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.