Like so many designers of clothing and costume Walter Plunkett trained in another field entirely. He studied Law, but soon found he was more interested in theater, so he shifted to acting, costume, and set designing. What makes him interesting is that the primary focus of his 150 film long contribution to Hollywood, was historical drama. He was without question THE go to guy if you wanted your film to look historically accurate.
The most egregious failure is also one of the films most important costumes, the red dress Scarlett is forced to wear to the birthday party. Apart from its vestigial bustle, it bears no relationship whatever to the fashions of the time. It is, however, a massively potent piece of theater, and as such a huge success.