The Germans - S1-E6
Other mistake: When Basil shuts Manuel in the burning kitchen and slams the door, the walls shake.
The Builders - S1-E2
Other mistake: In the end credits, Major Gowen's surname is incorrectly spelled "Gowan."
Fawlty Towers (1975)
2 other mistakes in season 1
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)
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.
Trivia: Sybil's friend, Audrey, who Sybil is frequently seen talking to on the telephone, has only one on-screen appearence in 'The Anniversary'. She is seen at the very end, when Sybil has just walked out on Basil.
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.