Factual error: In the scene where there is a train passing, the cars are marked "Burlington Northern." There was NO Burlington Northern until 1970, 7 years AFTER the shooting.
Factual error: Morrison did finish film school in real life. He did not drop out as depicted in the film.
Factual error: Pierrepoint is depicted as having a London accent. In real life, however, he was from the north of England, and had a Yorkshire accent, which sounds completely different. This is proved in a BBC interview with him, dating from the 1970s.
Character mistake: We see in a flashback (so, early 1600s) the protagonist with his mother in Ruen, back in motherland France. She tells him she is praying to St. Joan (d'Arc; they are in front of a statue of hers). Joan d'Arc wasn't canonized until the 20th century. You could argue that in the eyes of the laymen she was already considered a saint. However, her son does not object and considers her a saint as well, doubling down on the statement even later on in Quebec when he does not have to humour his mother, and he is a Jesuit. He obviously should know better. (00:35:25)