In the 1999 movie Galaxy Quest aliens come to earth seeking the actors from a Sci Fi TV series to lead them to victory against their enemies. They had watched the TV show and had mistaken make-believe for “historical documents”.
This came to mind when Oscar pointed out in the comments to The Ministry of Ungentlemanly Warfare that medieval manuals taught all kinds of dirty fighting tricks, offering this image and this one as examples. One of the problems with chivalry is it comes from literature. It is make-believe presented as faux history. But somewhere along the way we, like the clueless aliens in Galaxy Quest, have mistaken entertainment for history and began to take it seriously.