Factual error: When Eisenheim is at the Vienna rail station, catching his train after the Crown Prince has died, the overhead electric cables of the modern railway are visible in several shots.
Factual error: When the narrator says Eiseinheim bought a theater, watch the houses on the background, on the right side of the screen. One of them has a visible modern antenna on its roof.
Answer: Although the story is fictional, some of the details are based on the life of Austrian Crown Prince Rudolf, only son of Emperor Franz Josef. The painting of the emperor which Eisenheim creates is an actual portrait of Franz Josef. (from IMDb).