Factual error: The weaponry of the Romans and their use is wrong (as they are in all Hollywood movies playing in the Ancient Mediterranean that I know): Instead of one spear, each legionary would carry two weighted javelins, called Pila (singular: Pilum), which had a long narrow iron head. The purpose of these were to throw them at the enemy before melee; if they did not kill their targets, the pila would get stuck in their shields. The head shaft would bend, making the pila useless for 'return' to their original owners, and with the added weight of the javelin, the enemies' shields were rendered useless as well. Following this, the Romans attacked with short swords (the Gladii; singular Gladius).
Visible crew/equipment: When the slaves roll the flaming ball down the hill, we see it strike several Roman soldiers. Look closely, they are actually stuntmen dressed in asbestos suits.
Revealing mistake: During the initial breakout of the Gladiatorial training school an armed guard is stabbed on a balcony and falls to the arena below. He gets up and walks into the doorway adjacent to where he had fallen, apparently satisfied that his part was completed.
Trivia: Marcus Publius Glabrus is accused of neglecting to erect moats and stockades for his camp - a fatal mistake no Roman commander would have normally committed. The standard procedure was to build a camp with these defense measures at any stop, even if no enemy contact was expected. In order to forestall lack of wood for the stockade, each legionary carried two stockade poles with him.
Trivia: There were some battle scenes which were filmed with real amputees and maimed extras, as Kubrick was keen to give them authenticity and convey the brutality of war, but these were removed when preview audiences were disgusted by them.