Audio problem: When Gromit is knocking on his bedroom door to get Feathers McGraw to turn the loud music off, his bangs are out of sync with the sound produced.
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.
More trivia for Wallace & Gromit: The Wrong TrousersMore questions & answers from Wallace & Gromit: The Wrong Trousers
Answer: It was added at the edit. During the animation, the animators just made little sprinkles on the characters so it would look like the rain actually hit them.