After Margret Mitchell's (author of "Gone With the Wind") husband saw the scene with all the wounded soldiers in Atlanta he is reported to have said that "if we had had that many soldiers, we wouldn't have lost the war in the first place."
In the scene when Rhett hands Mammy a glass of whisky, you can see her sniff it before drinking. When it was originally filmed, tea was supposed to have been in the glass, but Clarke Gable (Rhett) substituted real whisky as a joke. The actress playing Mammy downed the glass, not realizing what was in it. The scene had to be redone, with tea, and obviously the actress didn't trust Gable after the first incident.
In the scene where Scarlet goes to look for Dr. Meade to help her with Melanie's delivery she goes to the hospital and encounters all the wounded soldiers. For this scene, the filmmakers were unable to get enough extras so for the injured soldiers lying in the distance they used dummies. The extras lay beside the dummies and by using strings, the filmmakers were able to move the arms and legs making it look like the dummy was moving.
The Burning of Atlanta scene was shot long before filming started on Gone with the Wind and indeed before either of the actors were cast in the role of Rhett and Scarlet. The purpose of this was to clear the MGM lot so the sets for the movie could be built. The buildings being burnt were sets from other films, the most noticeable being the huge gates featured in the original King Kong movie. The two actors in this scene were simply stunt doubles who doubled for Vivien Leigh and Clark Gable.
Cammie King, the child actress who played Bonnie Blue Butler, could not stop blinking during the filming of her funeral scene, when she is obviously supposed to be dead. A mask of her face was made, and the scene was finally filmed with the actress wearing the mask.
Another story about that famous "Frankly my dear ..." line is that the censors agreed to permit it only if the emphasis wasn't on "damn." This is probably one of the few times censorship improved a movie.