Deliberate mistake: During the fire Marge runs back into the house and goes to the kitchen cabinet that has a "keepsake cabinet" nameplate and a keyhole, in order to remove the wedding video, but the nameplate and keyhole didn't exist before. (00:33:35)
Deliberate mistake: Main headlines in newspapers appear translated for each country for the kids to understand it, but the rest of the news appear in English, despite the movie taking place in Paris. Also, most signs -such as the poisons in the window shop- have labels written in English.
Suggested correction: By that logic, it's a mistake that the characters speak English. It's an American movie set in France, and like you said, it's a kids' movie. Everything is in English for simplicity's sake.
Deliberate mistake: After Prof. Broom finally defeats the vampire countess near the end of the movie, many of her ghostly victims appear around the room. And, if you look closely, you can see that many of them look exactly like one another, revealing that the animators simply would repeat the same images over and over onscreen, to make the crowd bigger. (Confirmed in DVD commentary).
Deliberate mistake: Bender goes back in time to two minutes before Fry came back to the past. But if you time it, it was actually only 1 minute and 42 seconds.
Deliberate mistake: In the fire scene, Jon tries to catch Garfield from the chandelier at the main staircase. When Jon reaches out to grab Garfield, there is no rail on the stairs. It comes back in the very next shot & disappears again after Garfield says "Need a lift?"