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.
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.
Answer: It usually just means seeing something weird or unusual, perhaps something someone wouldn't believe you saw without proof. The phrase is usually "haven't got your gun", although some people replace "gun" with "camera." Basically think about a hunter spending all week in the woods looking for the biggest [fill in the blank] and the one day he goes out into the woods without his gun and then finally sees it.
Bishop73