It May Look Like a Walnut! - S2-E20
Character mistake: Rob is having a nightmare, part of which involves the disappearance of his thumbs. When he finds Laura at home, the script calls for him to say, "I want you to help me get my thumbs back," but he actually says, "I want you to help me get my thumb backs." His thumbs are also still there but hidden (you can see a portion of them in one frame of the episode).
Continuity mistake: When Benny comes to Top Cat's dustbin at the start of the episode, he first pushes a door bell and then uses a handle to knock on the dustbin. In the next shot, both the bell and handle have disappeared.