In the opening credits, there's a shot of a plaque with the letters "ECXXI" written on it. At first glance, they appear to be Roman numerals, but the letter "E" was never used to represent a number during Roman times. In fact, the only time the letter "E" was ever used as a Roman numeral was hundreds of years later during the Middle Ages, where it was used to represent the number 250, but it is no longer used in modern times.
The trainer blows a pea-whistle to call in the slaves. The first pea-whistle wasn't invented until the 19th century, although the Romans are known to use other whistles on board slave-galleys in order to keep the correct pace.
Antoninus is wearing trousers under his tunic when Tigranes enters the tent saying he arrived on horseback without any slaves. When Spartacus opens the treasure-chest Antoninus is suddenly bare-legged.
While in the senate house Julius Caesar is wearing the traditional white robe trimmed with a purple border, however when he walks out onto the steps with Gracchus he is wearing a gray robe trimmed with white vine leaves.
When Spartacus confronts Marcus Glabrus after the latter has been captured, the formed tears off Glabrus' medallion (which is the symbol of his office as commander of the garrison of Rome), breaking its chain. In the next shot Glabrus is shown wearing the medallion on its intact chain.
Referring to leaving Italy by sea, several characters, including Antoninus, refer to the port of Brundusium. The actual port, in ancient times, was known as Brundisium. The discrepancy could be regarded as a variation in pronunciation, until the midpoint of the film, when the pirate emissary points to a map of Southern Italy in Spartacus's camp tent, clearly showing the misspelled city of Brundusium. The other cities on the map - Metapontum, Capua, Tarentum, for example - are historically correct in the Latin spelling of their names.
At the beginning of the major battle between the legions of Rome and Spartacus's army, one shot shows the Roman standard in centre-frame, with another at the extreme left of the screen. The Roman standard would contain the letters, SPQR - Senatus Populusque Romanus - but the one in centre-frame here clearly shows SPOR. There is no mistake of this in the restoration released on Blu-ray. Strangely, the standard seen at the extreme left has SPQR, with a small mark at the lower right of the Q which may have been a prop-master's afterthought. One is tempted to believe that all the props were manufactured with SPOR, and some changed at the last minute. But not the one in centre-frame as the battle begins.
This is a romantic allegory of 20th century social ills, using an ancient setting to make a point (see trivia). It is not a historical documentary, nor a biopic. Though many of the main characters were real people, they are used fictitiously, as explained in the ending disclaimer. Most apparent errors in costume, custom, design, dialect, politics, armaments, etc., are specifically exempt from this list for the same reason.
During the final battle sequences the slaves drag down burning hay rollers. One of the slaves in Sparacus's army overshoots the end of the run and a Roman soldier generously drops his sword in order to catch him.
The goof items below may give away important plot points.
The length of Kirk Douglas' hair varies between scenes. When, after the final battle, he is captured and led to Rome, his hair is much longer than before the battle. But when he later fights Antoninus, his hair is short again.
In the fight scene at the gladiator school, when Draba has Spartacus against the wall with the trident at his neck, he looks to the audience to see if he should kill him, and receives the thumbs-down. The movie makes it evident that Draba is supposed to kill Spartacus. However, historically the Romans used the thumbs-down signal to spare the combatant (literally, to thrust their weapon into the dirt), and thumbs-up to kill them.