Revealing mistake: At the very beginning while the opening jingle is still going on, the camera cuts to Mr. Spooner standing behind the counter; he remains motionless, coat in hand, for a second or two before getting his cue to walk across the floor and begin the show.
Revealing mistake: When Mr. Humphries is talking to the canteen manageress about her lines as the hat check girl, in the shots of the former one can see Mr. Harman standing about in the background, waiting and watching for his cue to come on.
Friends and Neighbours - S10-E6
Revealing mistake: When Mrs. Slocombe's cat is supposed to be under the hat and "eats" the clockwork mouse, it is clear that it's a machine. This is most obvious when the hat spits out the mouse and slightly rises up in the front, which shows the wheels.
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.
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.