I've misgendered someone before, though in my defense, I didn't verbally call her a male, I simply had been assuming she was due to how boyish she looked, and her voice was quite androgynous, as well. If you've ever referred to a man as a woman, or a woman as a man, before they corrected you, you've misgendered someone! Horrors!
(really, though, it's not a big deal unless it's intentional and malicious. My story was about accidental misgendering)