Merlin uses magic to wash Arthur's cape in a bucket but when Gaius enters his room and all objects fall to the floor there is no water in the bucket.
In the first tournament fight, both Arthur and the other knight begin with their chain mail hoods down, and end the fight with them up.
When Arthur is fighting Valiant, footage from a different combat is spliced into the scene.
When Merlin is in the armory with Knight Valiant, he claims that he is getting Arthur's armor, and even picks up and leaves with the armor on the table. However, the helmet on the table is clearly not Arthur's as the front is entirely closed with only eye-slits and Arthur's has an open T-shape on the front. Later, in Arthur's chamber, he has the correct helmet.
When Merlin breaks in to Valiant's chamber, he uses his magic to draw a bolt on the inner side of the door. Since no one is in the room, the door shouldn't have been bolted from the inside at all.
Time index 3:15, Arthur asks Merlin "How's your mace work coming along?" The weapon he has in his hand is not a mace. It is called a flail, (involves a separate striking head attached to a handle by a flexible rope, strap, or chain). A mace is a club weapon (no rope, strap, or chain).