The Builders - S1-E2
Other mistake: In the end credits, Major Gowen's surname is incorrectly spelled "Gowan."
The Germans - S1-E6
Other mistake: When Basil shuts Manuel in the burning kitchen and slams the door, the walls shake.
Fawlty Towers (1975)
2 other mistakes in season 1 - chronological order
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.
More quotes from Fawlty Towers
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.