Amadeus
As well-loved as the 1984 film, Amadeus is, even the most hardcore of fans have to admit the number of liberties that the director took with historical details. Critics loved the film but many of them were put off by the inaccuracies. Especially how Mozart frequently appeared to be an imbecile.
The biggest error that people have noticed is that the costumes of the dancers use zippers. The fact of the matter is that zippers were not invented until 1918. This means that even if Mozart became older he would never have seen zippers in his lifetime.