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)
Continuity mistake: During Maximus's death scene at the end you can clearly see the entrance that Commodus and Maximus entered the arena through in the floor of the arena. After Juba is done burying the figures of Maximus's dead son and wife, you can not see the entrance anymore when he is walking away. (02:21:00 - 02:22:55)
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)
Continuity mistake: When Proximo is about to be killed and says "shadows and dust", the background scenery changes. It is obviously an alternate take from when he says the same thing before Maximus fights Tigris, used because of the death of Oliver Reed, but that doesn't make it any less a mistake. (02:12:15)
Continuity mistake: After Maximus comes out of the tunnel after leaving the gladiator quarters, he sees his friend Cicero. Maximus makes a bird noise to get his attention, Cicero sees him and in an attempt to warn him he yells "Maximus." He startles his horse which runs away but a noose is around his neck so he is pulled off the horse and left hanging there. As soon as the horse leaves him and he starts swinging his hands you can see they are not tied. Then when Maximus runs up to him and holds his feet up to try and save him his hands are tied up. (02:12:50)