There's a thing many supportive people do when they're first getting used to the idea that there's gender options beyond male and female, and it looks like this:
They meet a person who's neither, or maybe somebody they already knew comes out to them. Their immediate, trained, purely automatic reaction is to mentally assign this person a binary gender, usually the person's (assumed) AGAB. This is not their fault, it's a side effect of growing up in a heavily cisnormative society.
They then realize that they've automatically mentally assigned the person this gender and they consciously reject it, going "No, I know better, they are not that gender".
What happens next is that they'll train themself out of mentally assigning this person their AGAB, but this is where things get a little funny, bc their brain is still too used to the male-female-binary. So whenever they yank their thoughts away from the AGAB-box in their brain, there's a high chance they manage to accidentally land them in the other binary gender box.
And this is how you get people who are trying really hard to unlearn misgendering people accidentally calling an AFAB non-binary/agender/etc person "Mr." and "he" or calling an AMAB non-binary/agender/etc person "Ms." and "she", even when they know those aren't the correct pronouns & terms of address for that person. It's literally because they're rejecting that person's (assumed) AGAB so hard that their still-binary-programmed brain goes straight for the other binary gender option. It's not malicious and it's a completely normal part of unlearning cisnormativity and exorsexism that many people go through.
