During the second fight between Merlin and Arthur, the same dark-haired man is visible on the background, but in different outfits. When the boys just prepare to fight, he stays on the street just behind and is in a brown shirt and red vest. When Arthur falls and Merlin notices Gaius and stops fighting, the same man is again behind them inside the barn and is wearing a darker brown shirt only.
If you look closely at the upper piece of wood which is supposed to be keeping Merlin's head and hands securely in place while he is in the stocks and just before he is pelted for a second time is visibly loose which means Merlin could easily push the piece of wood up freeing both his head and hands.
The dragon refers to Arthur as the "once and future king" but that title only makes sense after Arthur has already been king and died. "Rex quondam" literally means "the king that once was".
When Merlin is put in the stock, the kids throw vegetables at him, mostly salad and tomatoes. Tomatoes were not known in Europe until Columbus brought them from the Americas in 1498.
Gaius wears glasses, yet the concept of spectacles was not thought of until the 13th century.
The scene where King Uther shares a meal with Mary Collins (magically-disguised as Lady Helena) shows Uther eating a modern strawberry. Such large breeds of the fruit were not available until the 17th century after a cross between the sweeter Virginia strawberry and a larger Chilean berry, bred in Europe afterwards.