When showing his mother his pre-version of the "La Goulue"-poster for the Moulin Rouge Toulouse-Lautrec is portrayed living alone in his apartment. The painting was done in 1891, when he actually lived with his friend, Dr. Henri Bourges in the Rue Fountaine 21.
When Henri Lautrec arrives at the gallery for the showing of his pictures, as he 'walks' in, his shadow on the ground clearly shows José Ferrer's legs tucked behind him as he walks (on his knees).
When Henri falls down the stairs toward the end of the film, his legs suddenly appear regular-sized.
Milan I. of Serbia has introduced himself, by giving his card, as "Milan IV, King of Serbia". The movie is set from 1890-1901, while Milan was only King of Serbia until 1889. Also his card credits him as "Milan IV", on which name, fully "Milan Obrenovic IV" he was the prince of Serbia and only as of 1882 as Milan I, he was King. During the time of the movie's setting Aleksandar Obrenovic, aka Aleksandar I., was King of Serbia.
When an actor is swinging on a chandelier, a crew member is seen pushing him.
Henri de Toulouse-Lautrec mentions that Antonio de Correggio died before turning 40. Actually Corregio was 44 when he died in March 1534.
Henri is obviously older than his mother.
Henri's nose is clearly larger when he is a teenager than as an adult.