Pompey and Crassus are made "consuls of the Roman Empire". But there was no "Roman Empire" until Augustus was named Emperor by the Senate. At the time, there was only the Roman Republic. The proper title would have been "Consuls of the Republic".
At the end Caesar proposes Crassus and Pompeius made "Co-Consuls". Consuls were *always* elected in pairs in the Roman Republic, and they were *elected*, not chosen. When Agrippa says "Never in the history of the Republic" he's cut off before he can say what we think he says, but it's not true if he were going to say that Consuls never came in pairs. They always did, even in Imperial times.
In the charge that precipitates the climactic battle, the legionaries of the Roman front line are shown advancing with shields in the left hand, swords in the right. Except for one in the lower foreground: his right hand is empty, he is advancing into combat with only a shield. As soon as he makes contact with a member of the rebel army, he falls on his back even though he hasn't actually been attacked yet.
Spartacus kills a gladiator in the arena by slitting his throat after their fight. The dagger, clearly visible in the following shot, is clean.
Cinna has the scar over his left eye. When he runs out to fight Spartacus at the start of the slave rebellion, the scar is over his right eye. When he falls to the ground, it is back over his left eye again.
In the final battle, the Roman army is shown marching downhill. The grass ahead of them is already trampled from previous takes.
When Spartacus is about to cut Crixus's throat, the fillings in Crixus's mouth are visible.
When it is revealed to Spartacus that his wife is pregnant he picks her up and yells, "I'm gonna have a son." This is obviously dubbed as his jaw fails to move at all
When Varinia and Flavius leave Rome, they pass the crucified slaves. The crucifixions took place along the Via Appia, which runs south from Rome. But because they are headed for the Alps, they would have left Rome in a northerly direction.
When Spartacus is fighting in the arena with sword and shield he has his sword in his right hand and shield in his left, except in one brief shot where the film was obviously reversed.