The most obviously case is George C. Scott/Patton. You see real life newsreels of Patton, and my reaction is always, “Wait a minute, that’s not Patton!”
I saw the movie Patton before I'd ever seen a photo of the real Patton, and was disappointed.
But now I think the real truth is, the real Patton was a far more interesting character than George C. Scott's caricature could ever hope to be.