Factual error: Toshiro Mifune's character, Kikuchiyo, travels to the bandits' camp and steals one of their two remaining fuselock muskets. Shortly afterward, the bandits fire twice at the samurai within five seconds with their last firearm. A fuselock takes as much as two minutes to reload, prime, and fire.
Deliberate mistake: When Kikuchiyo impresses the Samurai by catching a fish, it's rather obvious that it's a dead fish that he grabs out of the water. A live fish would flop around and be gasping for water, but the fish he holds is completely motionless. Obviously this was done because it actually is QUITE difficult to catch a live fish in water.
Trivia: In some instances in the film, such as when Gorobei kills the bandit who was checking the water level with an arrow, the actors were actually shot with real arrows. The actor would simply wear a block of wood under his costume (which you can see if you know what to look for) and an expert archer would actually shoot him.
Trivia: Right after the battle scene where Kambei kills a bandit with a high stance strike, there's a shot of a bandit being swamped by peasant militia. Pay attention to another bandit riding up into this shot in the upper left corner. There's an unsuspecting extra who is standing right in the path of the second bandit's horse. The horse rams the extra, violently throwing him to the ground.
Trivia: Seiji Miyaguchi, who plays Kyuzo the master swordsman, actually had the least experience with swordplay among all of the actors who play samurai in the film. In fact, he never even touched a sword until a few weeks before shooting the film. Most of his fight scenes rely on clever editing to make him appear to be a master.