The sticker Julia puts on Thibault changes position between shots, and in some scenes is gone altogether.
When the Earl of Warwick speaks with the witch, the wart on the witch's chin is on the right of her chin. It then moves to the left, only to then move back to the right.
Julia's shoes change between shots when she first meets Thibault.
When Julia is at the castle, she is first wearing black boots, then white sandals.
Julia takes off her ring to show her family's coat of arms, but in the previous shot she was not wearing it.
Thibault identifies himself to Julia as the Duke of Anjou. Thibault is from the 12th Century; the title Duke of Anjou was created in the 14th Century.
The end sequence where they are on horseback and being chased by police it's clear that Julia has been replaced by a mannequin dummy in the majority of scenes.
Thibault is from 12th century France, but the costumes are from the early 15th century.
During the chase scene, the police say the suspects are heading for "State and Michigan." In Chicago, State St and Michigan Ave are parallel and thus there is no intersection of State and Michigan.
It's not explained how the Wizard pays for his room, the laboratory he sets up or obtains the pig penis he puts into the formula.
Andre and Thibault not only go forward in time, but end up in another country as well.