In the scene where Gwenhwyfar and Lancelot kiss in the chapel, the cross on the altar is draped with a rosary. The film is set in the fifth century, but the rosary did not exist until the early 13th century.
Elaine tells Gwhenhwyfar she is the "happiest girl in England". The film is set in the sixth century, but the country was not known as England until around the dawn of the 11th century. Elaine would have referred to the land as Britain or the Roman form, Brittania.
The portrayal of Scotland is inaccurate for the time. Before the Saxon invasion, Scotland was made up of barbarian tribes known as The Picts, whom were rivals with the Britons, not allies. They would not have lived in castles.
When bandits ambush Guinevere and Lancelot, Guinevere and her horse are pulled down to the ground by two men. After focusing briefly on Lancelot fighting the bandits off, we return to Guinevere and see only one man pulling her away. After another Lancelot shot, there are two men again.