Plot hole: The ages of the children on the show are really inconsistent. When the show starts in 50 BC, Octavian is around 12, Lucius is an infant and Vorena the Younger is at least 8 (given that she must have been concieved before Vorenus left for Gaul). Three years later Caesarion was born. When Simon Woods takes over the role of Octavian, Octavian is around 19, as stated on the show. Lucius should be seven but looks like he's four and Vorena should be fifteen but still looks eight. Episode 9 of season two takes place in 32 BC, 18 years after the first episode. Octavian might very well be 30, but Lucius (who would be 18) is around seven, Vorena (who would be 26) is a pre-teen and Caesarion (who would be 15) is around eight. Even if the events were moved up so that episode 9 is actually set earlier, the ages of the children still don't match when compared to how much the other characters have aged.
Continuity mistake: Pullo has a hole drilled into his head after his return to Rome, yet in later episodes there is no visible scar, even in scenes where his hair has been shorn.
Factual error: The future emperor Augustus is called Octavian, an English modification of Octavianus. However, before being adopted by Caesar (which happened after Caesar returned from Egypt) his name was Octavius. When he was adopted he took the name Gaius Julius Caesar Octavianus, and at that point the English version of his name is Octavian, but before the adoption he did not have that name. On the show they are thus calling him by a name he did not have at the time.
Factual error: The eagle was the standard of each legion, not Caesar's personal standard as stated in the series, even by Caesar himself (to Brutus). Also, the Aquilifer (eagle bearer) was traditionally bareheaded - unlike the other standard bearers, he did not wear a bearskin or a helmet.
Factual error: At Atia's house, Pompey bores his hostess with stories about how he fought the Parthians. The real Pompey fought against Pontus, Armenia and the Seleucids of Syria, but never against Parthia.
How Titus Pullo Brought Down the Republic - S1-E2
Audio problem: In the scene where Mark Antony and his men are attacked by a mob on their way to the senate forum, Mark Antony says, "Rally to me" three times. The first time he says it, his lips do not move. The third time he says it, the movement of his lips do not match the words.
Factual error: At the beginning of this episode, Pompey and Cato talk about the battle against 'the Illyrian pirates'. There were no 'Illyrian pirates' in Pompey's days. Those pirates had already been wiped out during the Illyrian Wars of 229 and 219 BC. The pirates Pompey did fight were from Cilicia, in the south of present-day Turkey.
Factual error: In this episode they make comments on how traitors will go to Pluto, meaning it in the sense that they will end up in hell. Pluto was not equal to hell; everyone who died went to Pluto.
Factual error: The verse Octavia recites is from Virgilius' poem "the Aeneid". That poem was written at least a quarter of a decade after the death of Caesar.
Revealing mistake: Kevin McKidd's Scottish accent appears every now and then. For example in this episode, when he says that Marc Antony "negotiates with a whore and a dwarf at his side".
Factual error: Vercingetorix was indeed displayed publicly in Caesar's triumph, but he was executed afterwards, not during the triumph itself.
Factual error: The 13th legion's symbol was a lion, not a boar as depicted in the series.
Factual error: Brutus proposes that Caesar be made imperator. The word imperator would come to mean emperor during the age of the Roman Empire, but it did not have that meaning during Caesar's time. Imperator was a military title which Caesar had been given long before the events in this episode. Even if the word is used with its current meaning it's an error, as Caesar never became an emperor. The word Brutus would have used is dictator.
Factual error: Caesar was not murdered on the Senate floor, as depicted in the series. That was the conspirators' plan, but when they learned that Mark Anthony was coming to meet Caesar, they instead lured Caesar into the portico of Pompey's theatre and killed him there.
Suggested correction: The reason the senate was meeting in a different place is because the Senate house had been burned down a little while before so they met elsewhere during its reconstruction.
Factual error: Octavian was not in Rome when Caesar was murdered.
Continuity mistake: When Vorenus ask Eranus where his children are, Vorenus raises both his arms. When it cuts, they are down.
Factual error: Cicero's secretary, Tiro, is seen throughout the series wearing a plaque around his neck marking him as a slave, and here Cicero tells him, "You've been freed in my will". However, Tiro was freed in 53 BC, several years before the events depicted in the series.