In the episode when the gold truck is passing through Mayberry, two "G Men" from the Treasury Department come to the courthouse to talk with the Sheriff. Since Barney is the only one there, they sit down to wait for Sheriff Taylor to return. When Opie comes in, he walks past the two G Men, and they are two totally different men, and they are both now wearing hats, which the first two were not. You only see them for a second, but it's obvious it's two different men. When Andy arrives, it's the original two again, without hats. (Extra note: one of the original two who came in, and had a speaking part, was played by Rance Howard, the real life father of Ronny Howard, who played Opie.)