Revealing mistake: When the gentleman buying the dress comes out from behind the curtain in the change room, you can see the set for the canteen, with the lights off. (00:14:45)
German Week - S3-E6
Revealing mistake: When Mr. Humphries and Mr. Lucas make their toast with the German mugs, Mr. Lucas' mug breaks but Mr. Humphries' does not. A moment later, when Mr. Humphries notices the mug has not broken, you can see that he crushes the mug with his hand.
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.
Answer: In minstrel shows there was also a stereotype of black men preying on white women, so that's probably why she was not in black face. Also, Mrs. Slocombe is in blackface because she is an older, stout woman and she fits the "mammy" stereotype.