OK, let’s all acknowledge one thing first. For many transgender people, the desire to forget (and for everyone else to forget) that they ever were another gender is deeply satisfying and self-affirming. When someone slips up it is like a bucket of ice water hitting them, whether done by accident or on purpose.
Now, just like with a bucket of ice water dumped on someone by accident, what is the right thing to do? For the caring person causing the accident, it is to apologize. For the person hit with the ice water, it is to catch their breath and accept that it was an accident, point out how they feel about it, but not counter-attack — ever. And this is because any counter-attack sets them back. Even when it is not an accident.
All but a few transgender people understand this, but the few that don’t are the ones we hear from and it hurts all transgender people more than anyone else.