Near the beginning, when Sybil goes into the office after speaking to Basil about the date, the lights are obviously on in the office, but when she walks out, it can just be seen on the right that the office is in complete darkness, and then when the shot changes back to the regular one in the lobby, the office is lit up again, without anyone going in there in the meantime.
When the guests are chatting the lounge, Basil says, "It's best for her to be on her own. Now what is so peculiar about that?" When Roger says, "Her driving around in the town," Arthur has his pipe in his mouth. In the next shot, Arthur's mouth is empty.
Some viewers believe Manuel refers to the fish and seafood stew paella as a "pie." However the double 'l' in Spanish is pronounced like a 'y', so when Andrew Sachs (Manuel) says paella he is pronouncing it in a genuine Spanish accent which sounds like 'paeya', he is not saying pie.
Basil and Sybil's bedroom is clearly different from the bedroom they are using in The Wedding Party (1975). However, four years have passed, and the bedroom could have been redesigned.
When Basil drags Polly up the stairs, as he turns the corner, the carpet on the stairs bunches up - obviously not properly fastened to the stairs.
At 11:38, when Basil is trying to convince Sybil's friends about her illness, Virginia (a friend of Sybil) says: "Her face is puffed up, she's lost her voice, and her legs are a bit..." to which Basil replies "...Expanded. Sad, isn't it? Poor old sow." Right at this moment a microphone on the top left corner is visible.
The anniversary party is meant to be at night, with Paella for dinner. When Basil comes out of the kitchen after the exchange with Manuel and Polly, Ms Tibbs and Ms Gatsby greets him with "Good Morning Mr. Fawlty" to which he replies naturally "Good Morning".