I can't believe no-one has written this already!
At the beginning of the movie, in Germania, Maximus is seen riding a horse with STIRRUPS. Also later on in the film when the soldiers ride into Maximus' home they too are riding with stirrups. The Romans DID NOT have stirrups!! I do A-Level Archaeology and I have done a project on the Roman cavalry so I know what I'm talking about! Stirrups didn't appear until much later towards the end of the Roman Era when the Mongols developed stirrups.