Robin Hood (spoiler alert)
My most favorite of these by a wide margin was the inclusion of a 12th-Century version of "Higgins boats" that the French king used in the movie to land his troops in his invasion of Britain. Of course, Higgins boats in the 12th Century were, appropriately enough, made of wood and rowed by oarsmen.
If you don't know what these craft were, Higgins boats are thoroughly modern inventions used to deploy troops in 20th century amphibious assaults. They were used widely throughout WWII, including, of course, in the Allied invasion of France on D-Day. (So the fact that the French were using these to invade Britain seven centuries earlier is all the more amusing.)
I just about fell out of my chair laughing when I saw the French invasion army landing in a flotilla of these in the movie.
And, of course, there were a lot of other anachronisms as well -- thoroughly modern political sensitivities and gender roles not being the least of these.
Now don't get me wrong. I'm not at all a purist in these sorts of things in film. Still, for all the pretensions this movie ostensibly has to not being Errol Flynn's Robin Hood, at the end of the day Russell Crowe's Robin Hood is not one whit more serious than its earlier cousin. That's not a problem, however, if this movie is received as the piece of thoroughly enjoyable fluff that it is. The only difference is that Errol Flynn knew he was making a piece of thoroughly unserious fluff. I'm unsure that Russell Crowe is quite as self aware.