Factual error: There's a scene where Belle's father is riding away in the snow on his horse - the tracks left behind by the horse are exactly backwards. The round part of a horseshoe is at the front, not the back. (This was a deliberate 'in joke' done by the animators.)
Suggested correction: It is Belle who rides the horse in the snow, not Maurice. If we're talking about this being a mistake, then it could be a deliberate mistake, but if this was truly done as an in-joke, then I think it falls more under trivia. The tracks being backwards is factually incorrect, but it doesn't really fall under the category of "mistake" if it was deliberately done as a joke.
Factual error: The movie is set in the Middle Ages. Therefore, the traders who come to the Winterland to sell their goods must be quite ahead of their time to carry fairly modern lighters. (00:04:20)
Factual error: At the end of the Mamushka, Fester catches the final knife down his throat. With the length of the blade and the angle at which the knife is in his mouth, the point of the knife should be coming out the back of his head.
Factual error: A football game is shown playing on TV at one point. Impossible as the movie is taking place in spring - football games, both college and the NFL, are played in the fall/winter.
Suggested correction: This is during Jack's delirious dream and he wrote it that way to distract Carter who was waiting for the cable repair man. It wasn't something that was meant to be happening in real life.