Continuity mistake: Julia (Christina Applegate) puts a "Hello My Name Is..." sticker on Thibeau's tunic with her phone number on it. Only part of the sticker is visible. When he turns around to walk down the stairs, it is fully visible. A moment later when he stops the purse snatcher, it's gone. Three seconds later, talking to the woman dressed as a witch, it's back again. (00:50:15)
Revealing mistake: When they are driving back from the museum Thibault and Andre are both feeling sick and Julia asks Hunter to slow down. Hunter mentions that they are only doing 22mph and Julia replies "can you try going at 21?" However in the background a passer by is jogging past them - that's a pretty fast jogger.
Continuity mistake: Right at the end, when Julia is at the castle that she got, when she is walking by the cannons you can see that she has black boots on. But in the next shot when the old man is talking, if you look hard enough in the last frame of that view, she is wearing white, high heel sandals.
Deliberate mistake: Just after arriving in the 21st century, Thibault and André enter an elevator and scare away its occupants. Attempting to chase them, the elevator doors close despite their attempts to keep it open. Elevators have a safety mechanism which opens the door when they encounter resistance. This was ignored deliberately in the film for comic effect.