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.
Revealing mistake: At the last battle Katsushiro stabs a bandit who's climbing the fence to escape. It's completely obvious that the sword never penetrates the bandit. In fact, Katsushiro just seems to just tap him with the tip and his clothes aren't even torn from it, yet he still dies. Maybe a heart attack?
Plot hole: In the scene for Heihachi's funeral: Gisaku is at the funeral and is seen full face as Kikuchiyo shouts the bandits are coming. Everyone runs for their posts and bent over Gisaku is left standing by the grave. Kikuchiyo runs to dismantle the bridge with his crew when Gisaku's son, his wife and baby come by to get the old man who they say has refused to leave the mill. There is no way Gisaku could have outrun Kikuchiyo to the bridge and there is no way that his son and daughter-in-law could have known he was in the mill"