Duncan uses the word "assassin," which would have been unknown to someone in England at that time, as it's an Arabic word that wasn't coined until hundreds of years later.
The prisoner writes on the wall "Walk in love, walk in service, and you will walk in honor".
These words would not have been written like that and they wouldn't have been chosen in the time of King Arthur (ca. 600). In those days, people spoke Common Britonnic.
Among MacGyver's charges against Galahad, was asking who called for the Council of Judgement against Merlin. But as already established, a Council of Judgement would have been called anyway, as it was the law of the land in Camelot, so somebody would have called it had Galahad not done so.