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
Gourmet Night (a.k.a. Gourmet's Paradise) - S1-E5
Revealing mistake: When the Colonel first meets Basil, the same shot is used twice of the Colonel twitching.
The Wedding Party (a.k.a. Sex) - S1-E3
Basil: Did you ever see that film, "How To Murder Your Wife"?
Major Gowen: How To Murder Your Wife"?
Basil: Yes, awfully good. I saw it six times.
More trivia for Fawlty Towers
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.