I only did it once.
We are still friends, I still try to call him her and she. I used to know her when she was a guy. A real nice one too, she used to stand up for me all the time.
Well, that's good. I mean, I've slipped more than once but the guy with whom i did is kinda chill and didn't even comment on it. But it does make sense that someone would get mad, and it's not about being treated specially or different. It's about being treated, like you said, as a human.
If it's not clear what I mean, a human should have the right to establish an identity of their own. They have the right to know what they like, dislike, believe, feel, and of course, what their gender is. An important part of gender is one's pronoun, and denying someone's ability to decide on that is pretty indecent. Of course, I'm not referring to people who want to be called "princesself" or something, since "princess" isn't even a gender. I guess that was part of the point you were making earlier? Most of the weird pronouns, though, come from the lack of a commonplace, gender-neutral pronoun. Most people use "they," but other people get confused by the plurality of the word so??? but the point is misgendering someone unintentionally does warrant some anger, but not like "wow you're literally satan" kind of anger.