Now this is just sad. The actors who were struggling with the heavy costumes - Ray Bolger as the scarecrow, Jack Haley as the Tin Man and Bert Lahr as the Cowardly Lion, were given the short shrift when it came to lunch on set. They weren't allowed to eat with the rest of the cast.
As the story goes, this was a precautionary measure in case their appearance terrified any of the MGM workers. In an age before prosthetics or heavy makeup of the kind employed in this movie were a regular sight, to the untrained eye the three of them looked like real life monsters as opposed to the adorable characters we saw on screen, and therefore were deemed unfit to be allowed to mingle with normal people. We imagine that this kind of segregation did wonders for their already battered morale.