Factual error: When a group of soldiers goes to Maximus' villa to burn it and kill his family, his son points them, saying in Italian "Mamma! I soldati!" ("Mom, the soldiers!") and then "Papà!" ("Daddy!"). This is because the young actor (Giorgio Cantarini) is Italian and they didn't translate, for some reason. As a result, he's speaking Italian in a movie in English, where people are supposed to speak Latin, in a province where Italian was never ever spoken. (00:43:07)
Factual error: When the execution squad is ready, Quintus says "fire". Of course this is incorrect, since you don't fire a bow and arrow, but "loose" it. The term "fire" came only with the invention of firearms. It's a common mistake in bow / arrow type situations. (Extended version only). (01:47:10)
Factual error: The opening battle is wildly inaccurate. The Roman legions were trained to fight as a regimented force, and to maintain formation for mutual support. In the film, the formation collapses instantly upon contact with the enemy. Further, the Roman legions used spears called pila. Doctrine called for them to be thrown while the enemy closed. The Romans would then draw their swords and fight, while remaining in formation. Though the Romans are shown holding their pila in the opening scenes, they are never used against the barbarians, and we see no pila-riddled shields and/or corpses in the background. (00:09:15)csteel310
Factual error: At several points in the movie you will see speeches to a large crowd. These take place at the Piazza San Pietro. This square however is late Renaissance. Connected with this is the columns you see in these scenes. They were designed by the Italian artist Bernini, and the film crew didn't even remove the statues of all the popes.
Factual error: Architectural elements were from several centuries later. Note the balustrades, bell towers and domes in the Rome skyline shot and, more than anything else, Maximus' house, a typical 15th century Chianti villa. Also the columns are not painted, which they were in ancient Rome. (00:43:10)
Factual error: The last gladiator to be killed when Maximus is chained to Juba holds a trident. This kind of gladiator is called a "retiarius." They're also the ones who fight using nets. The shoulder armour was always worn on the left shoulder, not the right, because left-handedness was frowned upon in Ancient Rome. Also "retiarius" never wore face masks for helmets as it would give them too much of an advantage. Every Gladiator was given equal advantage depending on their specialty. (00:55:25)
Factual error: There's a blackboard in one of the scenes when there's about to be a gladiatorial contest. The Romans may have had blackboards then, but the writing on it is wrong. The sort shown takes time to chip into stone used on statues, etc, and I suppose it would take quite a long time just to write things on a blackboard. The Romans had different writing for things like that, as shown in the graffiti in Pompeii. It's more flowing, and I expect it's quicker to produce. (01:15:00)
Factual error: When Maximus walks through the wheat field it is obvious (to a botanist) that the wheat shown is an octaploid variety which was developed in the 1950's for the Green Revolution. The type of wheat grown during Roman times can still be found in many areas too, so it's not like the filmmakers can't find it. (02:19:45)
Factual error: In the scene where the two senators are quarreling about the note about the gladiators, we can read: "gladiatores violentia." This is an encroachment on latin grammar. Since gladiator is a male word, a corresponding adjective should be male too. Also, violentia is a noun. The correct form would be 'gladiatores violenti = violent gladiators' or 'gladiatores violentae = gladiators who fight like girls' depending on which school of thought you ascribe to (the latter being the claim that it is an informal use of Latin, insulting the gladiators by slighting their masculinity, accusing them of an effeminate form of fighting.).
Factual error: During a meal in the gladiatorial school, Juba is shown eating with a metal spoon. Slave-gladiators almost certainly only ate with their fingers, wooden spoons, or wads of bread to sop up their food. They never would have access to metal spoons that could be sharpened and used to attack the guards keeping watch over them. (01:36:20)