This is in the scene where Aragorn goes to summon the army of the dead. He is holding a lantern in his left hand and a sword in his right. The King then calls forth his army and Aragorn turns around to look at them, and the lantern is now in his right with his sword in his left hand. The camera finally cuts to behind Aragorn and the lantern is back in his left hand, with the sword again in his right hand.