During the show actual (real) people were mentioned. I believe there is a list somewhere but I can't find it. So under this topic list the real person and the episode if you know it.
I'll start with Glenn Ford in "The Bank Job"
I know there are lots of others.
Barney mentions bandleaders Cole Porter and Ted Weems.
Gangsters Pretty Boy Floyd and Al Capone.
Vaudeville comic Joe Penner.
In "Back to Nature", Gomer tries to back out of the camping trip saying "Well there's this movie on TV tomorrow night with Preston Foster I wanna see".