Factual error: Women were not allowed to be mixed with men. Only Vestales could stay where men were. Women could see the games just from the last ring-level of the Colosseum.
Continuity mistake: When Maximus has escaped being executed in the forest, he throws a sword into a soldier. Slow it down, the sword is actually already there in one shot, then it appears suddenly. Also the sword penetrates the left side of the body near the heart, yet when we see the soldier falling from the horse it's in the middle, then back to the left side. (00:39:00)
Suggested correction: According to this site's rules, if you have to use slow motion to see something, it's not a valid mistake.
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.).
Suggested correction: It's a flyer advertising gladiators and violence: two distinct promises, however intrinsically related, as indeed the violentia is to be expected from the gladiatores. But I can see no indication that there was any intention of building a complete sentence; it's the just two nouns thrown together with a picture, and however anachronistic the flyers be in the movie, it's a kind of semantic construction that would have worked just fine for Romans then as it does for us today.
Factual error: Early in the film Maximus walks through a cereal crop trailing his hands against the heads of grain. Except that dwarf varieties of cereal were only bred in the 20th century. Before this a crop would have been up to a man's shoulders.
Suggested correction: No doubt the scene was shot with a modern variety, but it turns out that it is a good approximation of the wheat grown at the time of the story. Two varieties of wheat grown in ancient Rome, the Emmer and Eikorn varieties, reach only 2-3 feet at maturity. These were originally wild wheats that had been cultivated for 8-10 thousand years BCE.
Suggested correction: This movie is not based upon facts. Many element were fictionalized. This is just example of fictionalized things.