Revealing mistake: Whenever somebody closes a door, leans against a wall, or goes up/down the stairs and brushes against the wall, it wobbles, revealing that the walls are lightweight stand-up props.
Fawlty Towers (1975)
1 revealing mistake in show generally
Starring: John Cleese, Connie Booth, Andrew Sachs, Prunella Scales
Genres: Comedy
The Builders - S1-E2
Deliberate mistake: When Basil is arguing with Polly and eventually decides to call O'Reilly, he trips over the gnome. However when we see Basil come out from behind the desk, he takes the gnome out of a side gap in the desk, perfectly out of reach of Basil tripping over it. (00:14:30)
More quotes from Fawlty Towers
Basil the Rat (a.k.a. Rats) - S2-E6
Trivia: Melody Lang, who plays Mrs. Taylor in this episode, is Andrew Sachs' wife.
Communication Problems (a.k.a. Theft) - S2-E1
Question: What's the name of the piece of music played when Basil turns up the radio after Mrs Richards says it (the radio) doesn't work?
Answer: It's "Leisure Complex" by Dave Gold and Gordon Rees. Can be found on YouTube. The relevant clip is around 25 seconds in.