When Scarlett is attacked by some ruffians, on her way to her mill, note the colour of the horse. It is black. When she is attacked, you see a shot of the horse rearing up, ecept it's a distinctly ginger colour. Then the horse goes back to being black again. I guess they couldn't get the original horse to rear.
in the horse-rearing scene (with the color changing horse) the reason they used two different horses is because they took two entirely different shots. Vivien Leigh never actually filmed the scene with the horse rearing, it was a double with a wig, so the horse mistake wasn't because one horse wouldn't rear, but that they probably just forgot.