When Merlin tries to burn the Black Knight with magic, the knight is standing on packed ground with many footprints and there is a cobbled road nearby. The knight doesn't move at all, but when Merlin and Arthur, and later Uther look at him through the windows, he is standing on a smooth grass field with no cobbles or footprints visible.
After Uther kills the wraith, he takes his helmet off with his left hand in a close shot. Then we see him from a distance with the helmet in the right hand; his left hand is empty; his sword is on the ground a few meters away. Then in another close shot, he throws the sword from his left hand to the ground.
When Sir Owain accepts the challenge, the black knight says the duel will take place at noon. However, as Arthur and Owain are walking into the arena, the shadows on the ground clearly indicate that it is either early morning or late afternoon.