Factual error: One of the naked female dancers at the party has breast implants. Although some forms of plastic surgery were known to the Romans, breast augmentation was not one of them. If you want to check for yourself she appears just after the masked dwarf moves through the shot.
Continuity mistake: Near the start, right after Spartacus slices a man, you see him holding only the hilt of a sword - when shots of cutting are seen, the blades are normally added digitally, but it wasn't in this case. A couple of seconds later it is back to a full sword.
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.