Revealing mistake: The road leading out of the samurai village has distinct tire tracks from vehicles.
Revealing mistake: Before the final battle, just after Katsumoto says that the Imperial army will not surrender, there is a cut, appearing when the Japanese army are ready to attack. You can see, at the right side of the screen, a transparent man walking in front of the troops. Actually you can only see a pair of legs walking without any body. (01:50:15)
Revealing mistake: In one of the large battles (the last if I recall correctly), one of the soldiers in the foreground can be seen clutching his chest before an arrow hits him. It's only an instant before the arrow hits, but noticeable.
Revealing mistake: During the final battle, some samurai horsemen are lighting hay panels to create a smokescreen. At one point a horseman rides past two of them without touching them with the torch at all, and they still catch fire. Moreoever, they start to burn from the bottom, not where the flame could have touched them (which of course it didn't).
Revealing mistake: During the melee in the final battle, towards the end, Algren looks around, seeing the death around him (much of this is in slow motion). In one of the shots focusing on Algren, behind him, on the left side of the screen, you can see an army soldier using his rifle to fight in close quarters. As he brings his rifle up, it noticeably flexes, indicating that it's a stunt-friendly rubber prop, rather than metal and wood. (02:04:45)
Answer: The Battle of Thermopylae, see http://en.wikipedia.org/wiki/Battle_of_Thermopylae for more info.
Myridon