In the earlier scenes of the movie Rembrandt's son Titus van Rijn is portrayed as a small child who can walk and talk with no problem. However the scene is set in 1641/2 while Titus was born in September 1641, which would make him a child who is one year old or less. At that age children can't do all that.
The painting "The Man With the Golden Helmet" is not depicting Rembrandt's brother Adriaen, as described in the movie. Also even if it used to be attached to Rembrandt's works, it is not clear who actually painted it. Most people believe that a student of Rembrandt's painted it. Since 1968, 47 years after the movie's release, it is not declared as Rembrandt's work anymore.
In the movie Saskia van Rijn writes in her will that her money should go to Rembrandt after her death. However in real life Saskia wrote in her will that Rembrandt gives the half of her fortune to Titus and if he dies to Saskia's family.
In the scenes set in mid-1640s and later Rembrandt is meeting the painter Seghers. However Hercules Seghers died in 1638.
In the first movie scene a man said that Rembrandt died on the 8th October 1669. However this is the day when he was buried. Rembradt died on October the 4th 1669.
When Frans Banning Cocq approaches Rembrandt to ask him to paint the "Night Watch", he tells him that they are 17 persons. However they were 18 shooters in the company, which are also portrayed on the painting.
Geertje told Rembrandt that she heard that Peter Paul Rubens in Antverpen is now a millionaire and that he makes pictures how the people like them. The scene is set in 1644. Rubens however died in 1640. The million Guldens, which Geertje mentioned was only collected after his death.