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.
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)