Revealing mistake: In the Fredericksburg battle scene, just after the Union officer orders the men to fall back, there is a 1 or 2 second close-up shot of a Confederate soldier taking a hit and falling back. As he does, a modern wristwatch is visible on his left wrist under the sleeve of his coat. (01:49:49)
Continuity mistake: In the scene where the three Confederate soldiers are about to be executed for desertion, a blindfold is placed around the deserter on the right. It is a narrow strip of cloth covering his eyes. The camera pans to the firing squad and then back to the three men. The blindfold on the deserter on the right, is now much wider.
Continuity mistake: During the changing of flags at the school, Major Jackson is being told that his country and the country they live in are one. The gentleman telling him this clasps his hands together to show unity. When he clasps his hands together he holds them at about face level but when the camera angle changes his hands are now seen below his chest. (00:11:27)
Continuity mistake: When Lee and Longstreet are surveying the map of the area, the position of the compass changes without anyone touching it. (01:02:00)
Factual error: Throughout most of the movie, Lee and Jackson are riding the wrong horses. Traveller was a dapple grey horse with dark grey mane and tail, and Little Sorrel was a dark rust-colored horse with a white star on his face. In the movie, Lee is shown riding a white horse with white mane and tail. At the beginning of the movie, Jackson's horse is correctly depicted as a sorrel with a white star. Then Jackson's horse is switched to a red chestnut with white blaze and four white socks.
Answer: Yes, she did exist. Through books.google.com, I found a book "Cemeteries of Caroline County, Virginia: Private Cemeteries" by Herbert Ridgeway Collins, that confirms Jane did live, that she and Jackson were close, and that he arranged for her burial after her death.