Continuity mistake: Mr. Fox gets to his apartment building sporting a huge cigar. He briefly talks, or at least attempts to communicate, with the girl in the lobby, and the cigar is gone. The cigar is back the moment he opens the door to his flat and the goons jump him.
Other mistake: The end credits are plagued by the biggest compilation of spelling mistakes you'll ever see. The French cast in particular has 2 out of 2 actors with their names wrong (PallarOy instead of PallarDy, a female, Danièle, turned into a male Daniel) and so many errors in the technical cast to make some of them impossible to identify.