Continuity: 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.
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Merlin mistakes
Other: In the scene where Uther takes Excaliber from Merlin while preparing for his duel with the black knight, we see that Uther is left handed. 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.
Continuity: 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.