I constantly forget up the pronouns of my transfriends and they never get mad at me, even if I don't correct myself. It really just depends on how you're communicating and if you're being an ass about it. If you're doing it in a way that invalidates their personal identity because you refuse to believe that they're not internally what they appear on the outside, you're an ass. If you do it because you weren't aware of their identity or because you're used to calling them by another pronoun, they should be more understanding. Some people are just starfishs, and that's coming from both sides.
and i've actually decided to not have a preferred pronoun so that this isn't an issue when i actually get to alter my appearance