10. Braveheart - The White Car
One of the most famous movie mistakes ever, and one of the most elusive - spend enough time online, and you'll find that there are multiple theories concerning the appearance of the fabled Braveheart white vehicle. Either it appears at Murron's funeral, driving in the background, or it's seen parked in the background of various battle scenes.
Either way, it's a fairly conspicuous addition to the film, and one that pretty badly compromises the period setting.