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.
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.
Suggested correction: This movie is not based upon facts. Many element were fictionalized. This is just example of fictionalized things.
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.