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)
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)
Audio problem: When Young Mr. Grace arrives, the sound department misses his first "Good Morning", which is audible only very faintly towards the end. (00:15:35)
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)
Revealing mistake: When Mr. Lucas goes to "knee" the pair of pants, the pants already have the hole in them, and his knee doesn't rip anything.
Camping In - S1-E4
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)
Camping In - S1-E4
Other mistake: The episode takes place on March 2nd, 1973 (date of the newspaper shown by Mr. Mash), but in Mr. Rumbold's office the sale chart for the month of March is already fully filled up.
His and Hers - S1-E5
Revealing mistake: On the tape player that the sales girl is using, the reels are moving in opposite directions, the tape can't play doing that.
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.