Continuity mistake: In the beginning Gromit is pouring tea and there is a teacup and toaster near the pot. Gromit sets it down, gets the mail, and then comes back. When he comes back, the cup is much further away from the teapot and the toaster is on the other side of the table. No one has been in the room since Gromit left.
Continuity mistake: During the train chase, the remaining empty freight car that Wallace is balanced on precariously switches tracks and detaches from the train. And yet once Wallace has gone flying and landed on the shopping trolley, the car has mysteriously reattached itself to the train.