Factual error: During the final battle as Saxa starts using the ballista against the Romans in the front, the ballista is portrayed as a weapon launching multiple projectiles at once, while in reality it only launched one at the time.
Revealing mistake: During the final battle there is a dual between Julius Caesar and Gannicus. At one point Gannicus spins Ceasar around and he is shown to wear modern day underwear.
Continuity mistake: During the final battle there is a long trap between the armies, filled with spears and hidden, where the Romans fall when trying to get to Spartacus' army. Before the Romans pass into it, a messenger from the Roman army comes through on his horse without falling into this long continuous trap.