Other mistake: When Uther takes Excaliber from Merlin while preparing for his duel with the black knight, we see that Uther is left handed. And he is left handed while writing. He tests the balance of the sword with his left hand, and in fact wears his sword on his right hip in all other episodes. Once he puts his helmet on and begins his duel with the black knight he suddenly becomes right handed.
Revealing mistake: At the very beginning of the episode, as the knights are about to send Merlin to Camelot with Lancelot, the knights walk towards the camera. As they do, there's a patch of green screen the special effects department must have forgotten to remove between Sir Elyan and Sir Percival. (00:02:10)
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)
Visible crew/equipment: When Arthur goes to Dragoon to ask for help, when Dragoon runs out of the hut to turn back into Merlin, on the left hand side of the screen you can see a camera crew in modern clothes. [Fixed in Netflix version]. (00:23:00)