One that gets my goat is Captain William Bligh being treated as the villain in the famous mutiny on the Bounty, derived from people who get their history from schlock movies.
Bligh was a hero, and the greatest seaman who ever lived. No really, nobody has ever matched the feats of sailing Bligh pulled off. Based on purely feats accomplished Bligh is head and shoulders above everybody else in history who's ever put a boat in the water. And he did it while taking 1/12th of a pound of bread as the entire ration of food for the day for himself and his 18 loyal men (he was given 1 week's rations and stretched them to 47 days). Even people eating actual decent diets and with modern advantages cannot pull off the stuff that Bligh did. Did I mention that he did it with no maps or charts because he wasn't allowed any by the mutineers? Yeah, the man was straight up supernaturally good at sailing
But, says you, what's that have to do with him being a villain? He could be a great sailor and also an ass. Quite true, but Bligh is famous for one other thing in his career: being the nicest, kindest captain who ever sailed. He took great interest in the health of his crew, insisting on high standards of cleanliness and a regimen for healthy exercise. The logbook of the Bounty, when recovered, revealed that Bligh gave scoldings for misdemeanors other Captains had men whipped for, and gave whippings for things other captains hanged men for. Why the mutiny happened isn't clear, Fletcher Christian was a close friend of Bligh's and his personal protege, and Bligh noted during the betrayal that he had "Dandled my [Bligh's] children on your knee!" Fletcher himself is described in the records as appearing mentally unhinged, claiming that living on the ship was hell so there's that.
So yeah, a genuine humanitarian and the greatest sailor who ever lived, constantly painted as the devil by pop fiction. Irritating to say the least.