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.
Other mistake: In the scene where Barca is bleeding at the pool, Batiatus comes and grabs his hair. Barca abruptly moves before Batiatus starts to actually pull his hair.
Mark of the Brotherhood - S1-E8
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. (00:01:40)
Factual error: When Lucretia is attending the market she is wearing high heel shoes which did not exist at the time, and weren't invented until the 15th century.