Factual error: In the show Gaius wears glasses, except eyeglasses first came into use in the late 13th century while the legend of King Arthur is around the 5th or 6th century.
Continuity mistake: The two vials are identified as Valerian sleeping potion (clear liquid) and Henbane (yellow liquid, deadly poison). Gwen pours the yellow liquid in the king's cup, but it's supposed to be the Valerian potion used here that puts the king to sleep. Gwen later dumps the sleeping potion into his ear when it should have been the poison. Then at the end when the Merchant identifies the 2 vials he identifies the yellow as Valerian and the clear vial as Henbane. (00:18:35 - 00:24:25)
