Actually, for the time, not a bad slip-up. At the scene just before the barbecue, where people are arriving to Tara, just before it cuts to the next scene, horses and buggies are driving up the lane towards the mansion. As it cuts, some of the horses on the left become ghosts. This is because the mansion was matte artistry.
True true...most of Twelve Oaks and Tara was matte artistry to save on expenses for the production. They could afford two completely different plantation homes. Plus, most of the soldiers in the scene where Scarlett is look for Dr. Meade at the train depot are dummies being moved by the real people near them.