When Scarlet is in Atlanta, she comes out of the jail after seeing Rhett. There is a flag flying behind her that she walks by. You get a glimpse of it as she passes, and if you notice, the flag has 50 stars on it!
When GWTW was made, there were only 48 states, so the flag still wouldn't have had 50 stars on it, although it did contain more stars than current states for the civil war time.
In 1865 there were 36 states, however, the flag in this scene has 35 stars, 5 rows of 6. These flags are actually only missing 1 star, for Nevada, which became a state in 1864.