Factual error: During the scene where Mozart is being carried away in his coffin by horse and cart there is a brief shot where he is being taken down a muddy path and a farmer is seen with his cows. If you look closely to the left side of the screen you can see a large overhead power line. If you look even more closely you can even see the wires coming out of the pylon. The movie is set in the 18th century and obviously it obviously should not be there.
Factual error: At the beginning of the movie when Salieri is first talking to the priest asking him if he recognizes some of his songs, they cut away to an opera. A woman can be seen descending stairs where there appears to be smoke falling down the stairs, which would have to be dry ice. While it was first discovered/observed in 1825, it wasn't commercially available until 1925.
Factual error: In the scene where Constanza starts to undress in Salieri's rooms, it is revealed she is wearing a 1980s style strapless bustier with separate bust cups and lots of frothy lace. This is not historically accurate. Women wore corsets which shaped their torso into a conical shape, pushed the bust together and up, and they generally had shoulder straps. The corsets were also mostly unembellished, so as to keep a neat silhouette under tightly fitted dresses.
Factual error: The scene where Mozart is dictating the "Confutatis" to Salieri: though the real Salieri was, in truth, a very accomplished composer and musical expert, there is no way he could write down Mozart's melodies as quickly as he does in this scene. The ostinato string part is especially improbable - he writes down a combined 968 notes for four parts in less than 10 seconds. Obviously done for time purposes, but unrealistic all the same.