 Vivien Leigh (qv) was instructed to really hit  Butterfly McQueen (qv) in the scene when Melanie is in labor.  After many takes, McQueen started to cry, refused to be hit again,  and Leigh (who refused to do the scene in the first place) could not  go on with the scene. Someone in the back clapped while Leigh  pretended to hit McQueen from then on.