Factual error: In his fight against Dennis and the Jabberwocky, the Black Knight uses a flamberge, a very distinct two-handed sword (note the little "crescent" hooks on the blade). The only problem is that these swords are a German design from the 15th century, more than 200 years after the movie is said to take place.
Factual error: The movie takes place in a Medieval European setting. This general historical context is strictly observed, but with one exception: the potato. Potatoes are part of the so-called Columbian exchange. There were no potatoes in the Middle Ages.