Continuity mistake: When the Jesuit tries to kill King Louis, Louis grabs a long bladed knife and stabs the Jesuit in the chest, but when the camera pulls back, the knife is gone, and no one has made a move to pull it out of his chest. Only the blade that D'Artanian threw at him is there.
Trivia: After Porthos tries to hang himself, Aramis reveals he had already sawn the beam to prevent the hanging. His comment when the barn subsequently collapses, "I'm a genius, not an engineer" was written as a pun in the original French manuscript. (Je suis un génie, pas un ingénieur).