Revealing mistake: At the beginning of the Scottish dance when Captain Peacock is demonstrating the sharpness of the swords, he first cuts through the center of a newspaper page held up by Mr. Spooner. However, you can see that, just before the sword makes contact, Spooner actually tears the paper apart down the middle.
Continuity mistake: In the ruined photo taken of Old Mr. Grace and his nurse and secretary, the secretary is looking away from the camera and her leg is raised. However, in the scene preceding, she was looking at the camera and raised her leg after the flash went off. Also in the photo, both of Mr. Humphries' feet are in the air, though one was on the floor when the picture was taken.
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