Revealing mistake: Mrs. Slocombe enters her tent after telling Captain Peacock about the top window. Peacock then obviously uses that moment to stand on his tiptoes and peek in. You can see to the right of the tent the metal cart pushed by Larry Martyn discreetly peek out of the stage entrance and stop as he waits his cue to come on the set, to a rumbling noise. (00:13:35)
Other mistake: The gender of Mrs Slocombe's cat is always different in each episode. Sometimes her cat is a female and other times it is a male. (eg. In 'Mrs Slocombe Expects', her cat is female. In 'No Sale', when Mrs. Slocombe complains about her having to get up to have a bath at 6:00 in the morning, she says, "The gurgling of the tank woke him up.").
Other mistake: There is another counter on the floor, to the right of the stairs, that is usually staffed by two women besides Ms. Brahms and Mrs. Slocombe. But throughout the show when there is a on-floor meeting for the entire floor staff, these two are never included in the discussions.
Revealing mistake: When Mr. Lucas gives Mrs. Slocombe the radio mike, it slips down her dress out of reach; Mr. Harman says "It can't have gone down all that far," and he's right: The short cord of the mike is visible sticking out of the dress for quite some time until Mrs. Slocombe herself pushes the mike down (in the appearance of trying to fish it out).
Continuity mistake: Mr. Lucas says the figures for the previous Monday were 325 pounds, and at that point in the day the departments combined (170 and 150) were up to 320, five pounds short. However, later when Mr. Rumbold is talking to Mr. Grace about the sudden drop in sales, he says they are "still 20 pounds short" of the previous Monday.
Continuity mistake: At the end of the show, after they reveal the 'new' ladies department. Mr Grainger is not wearing his glasses, as Mrs Slocombe has them in her purse, the shot changes to Mrs Slocombe, then back to Mr Grainger he is wearing his glasses, and there is no way he could have got them from Mrs Slocombe.