Revealing mistake: During the slave revolt at the Gladiator training academy there is a wideshot of the arena (with the slaves/soldiers fighting in the background) where you can see a slave trying to climb up a banner to the podium above. A soldier reaches down and stabs the slave however you can see he doesn't actually stab him. The slave then drops down and does a death twirl which isn't very convincing either. (00:56:42)
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.
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).
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.
Factual error: The armor and shields of the legionaries are of a type not used until over 100 years later, and there are no transverse crests identifying centurions.
Continuity mistake: The man holding the parasol at the start: In the close up shots,he is right behind Batiatus, and in an overhead one he is further back.
Factual error: Several things are wrong in the gladiatorial fight between Spartacus and Draba. Spartacus fights as a Thracian (thraex), but his sword is wrong (it should be the curved sica), his shield is much too small and his helmet and shin protectors are missing. Draba fights as a retiarius, but that type of gladiator was only introduced under emperor Augustus, some 60 years after the slave revolt.
Plot hole: How did Glabrus know about Silesian pirates aiding the slave army's escape? He informed the senate of this and yet was not made aware of it at the time of his capture.