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)
Continuity mistake: All the castle windows on the outside of the castle are small squares yet from inside the rooms they're arched.
Continuity mistake: During the Treaty signing Merlin's cape collar goes up and down depending on the type of shot it is - close up the collar is up, distance the collar is down.
Continuity mistake: After Merlin drinks from the goblet he puts his empty right hand up to his face. However, when he collapses the goblet is shown falling out of his right hand.
