Apart from the very obvious fuckups of songs like The Haughs of Cromdale (which mixes up two battles decades apart to make a story about one that never happened) or Chevy Chase/The Battle of Otterbourne (two sides of the same fight where one major character is unhurt in one version and chopped to pieces in the other) the more important issue is - what never makes it into songs.

It seems that the more catastrophic a historic event the less likely it is there will be songs about it. There are a lot of sad songs from the Middle Ages but nothing about the Black Death. A lot of songs from the mediaeval Islamic world but nothing about the Mongol invasions. Quite a few songs about the smaller conflicts of WW1 but nothing from Russia (with by far the highest death toll of any combatant nation) and nothing about the pandemic influenza that killed more people than the entire war. A few songs and military tunes about the colonial wars of the late 19th century, but about the Leopoldian genocide (20 million dead), nothing. 20 million Chinese dead from Japanese militarism and nothing but a few Maoist rallying songs. Lots of songs about the relatively bloodless Irish war for independence and nothing but patriotic fluff about the partition of India (about a million dead).

The most important historic events are just too horrible to sing about.