Factual error: After the birth of the baby, James Stewart and "Doc" Witherspoon are discussing the course of the war. "Doc" relates that he has always lived in Virginia and that his son died at Gettysburg at Little Round Top. No Virginia regiments fought at Little Round Top - only Alabama, Texas, Georgia, and Arkansas regiments.
Other mistake: When Jimmy Stewart talks to George Kennedy about the taking of his son as prisoner. Right after they look at the map. Look over the shoulder of Kennedy in next few frames. You can see a white vehicle move across the horizon under the mountains. Looks like a white van or truck. With a HD TV, you can even see the dust trail as the vehicle moves left across screen.
Revealing mistake: In the scene where the boy is shot in the leg and the carried away by an unionist soldier you can see more union soldiers storm past them. The bayonets of the soldiers are obviously made of rubber as they are dangling while the rifles are moved up and down by the running soldiers.