Character mistake: When Melanie and Scarlett are talking with an (off-screen) wounded Confederate soldier, the soldier says he hasn't heard from his brother since Bull Run. Only Northerners refer to that battle as Bull Run; Southerners have always referred to it as Manassas.
Character mistake: When Dorothy and the others enter the Haunted forest, the sign reads "Witches Castle One Mile." "Witches" is plural. To be grammatically correct, it should have said "Witch's Castle," if one witch, or "Witches' Castle" if more than one.
Character mistake: David Niven and his father are playing chess, and to checkmate his father, David Niven makes two moves, which you absolutely can not do - the only time you can move 2 pieces is when you move your castle and the king at the same time (castling), but you can't do that when you are in check.
Character mistake: On the proclamation announcing a plebiscite on a proposed "monarchy", the adjective "monarchical" is misspelled "monarchial".