Other mistake: During the scene in the banquet hall, as Tormund says "most people get bloody murdered, they stay that way", look at Daenerys in the background - a disposable coffee cup is on the table in front of her. This brown cup with a white plastic lid is nothing remotely like the style of cups they're all drinking from in the scene. HBO acknowledged the slip up and digitally edited it out on streaming services, reruns and future DVD releases.
Other mistake: The dragon is shot down by three giant arrows, and everyone looks shocked, with no idea where the attack came from, even Daenerys, who's flying high alongside it. Then the enemy fleet rounds a small outcrop, revealing themselves as the attackers, but given her height she would have seen them a mile off and steered well clear.