Character mistake: When Mrs. Slocombe volunteers to sing while Captain Peacock plays piano, Mr. Humphries responds that he didn't know she could sing, even though the department staff has been involved in many musical numbers over the series.
Continuity mistake: If a character says their assistant number throughout the show, they are different each time.
Character mistake: Mr. Lucas got himself in an embarrassing situation 'straightening the seams' of Mrs. Slocombe. Humphries passes by saving him by telling him that Mr. Granger wanted to talk to him, but he does it by saying "Griffin wants you." Wrong name. (00:07:45)
Other mistake: When Mrs. Brahms is talking to Mr. Granger, Wendy Richards' voice cracks stifling laughter when she says "Mrs. Slocombe didn't like to take down your trousers without asking you first." (00:18:00)
Other mistake: Pilot episode Mr. Lucas is holding the mannequin together with Miss Brahms. Mr. Grainger walks by and is appalled by the state of undress of the figure and says "Well, it's hardly sufficient, is it?" You can see the camera getting randomly out of focus in a bad, noticeable way. (00:03:20)
Goodbye Mrs. Slocombe - S10-E1
Revealing mistake: At the end during the credits, Mr. Humphries and the Lady he played are onscreen at the same time and you can see the splice made to allow that to happen.
Factual error: 2016 special - You Can't Teach An Old Dog New Tricks: Mr Harmon comments on a perfume called Belle, which he thinks is called "Belly", "I wonder what the next one will be called. Bingo Wings?" The episode is set in 1988 and the term "bingo wings" didn't exist before the 1990s. (00:10:40)
Answer: That wasn't possible in 1972. Until recently squatting was a civil offence in the UK, not a criminal one, and removing squatters was a complex and time consuming business.