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)
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: 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.