Valjean's coat and cloak have dirt on them while he's drenching his horse at the White Sergeant, but are clean before and after that.
As the story is set in post-Napoleonic France, most of the written text is in French, even if the spoken dialogue is in English. However, the sign on the wall outside Marius's headquarters, reading "Students' Society Law Reform," is in English.
During the street fighting in the 1848 Revolution, Eponine is killed when she throws herself in front of the musket ball meant for Marius. She collapses, falling to the ground as Marius is overwhelmed by soldiers who beat him down. Moments later the scene returns to their two bodies lying on the ground. Even though Eponine fell first as she died and Marius fell after her, her arm is on top of Marius' shoulder.
French prisoners were not used as "galley slaves" after 1748. The shipboard scene, imitated by subsequent adaptations of Les Miserables, may result from a too-literal reading of Victor Hugo's novel. When with the Bishop, Jean makes a throwaway reference to his time in the galleys. In the 19th century, "galley" was French slang for any kind of prison, in reference to the defunct usage of these ships.
In 1800, Jean's trial includes a jury, and the defendant is summoned to take the witness stand and give testimony. These two elements were not part of French trials at the time.
(at around 26 mins) A chorus sings the hymn "Ave Maria". The hymn was composed by Franz Schubert in 1825. The scene in the film is set in 1810.
Though the costumes are fairly accurate, the young women have the pencilled eyebrows, drawn at an unnatural angle, of the early Thirties.
When Jean takes Cosette to the convent, she is eight years old. She is confirmed and leaves five years later, but instead of being thirteen she is several years older, of marriageable age.
Although makeup and gray hair are used to age Fredric March, in the 20 years during which the story takes place, Charles Laughton does not change at all.