Yup, I've been there.

A friend of mine from the first job I had after high school transitioned maybe ten years ago (a time gap of about twenty years).

She would get pissed that I would deadname her, and sometimes use the wrong pronouns, partially because I would meet other trans people through her and never fuck up. I kept apologizing, and explained that it isn't intentional, it's just decades of brain patterns being laid down with the old name. It took me damn near five years to totally switch over without an occasional fuckup.

It's a thing, it turns out. Even the most supportive, staunch allies have trouble switching those patterns because they're kinda etched into the brain. It happens when people make other name changes too. Happens with titles after promotions, with surnames after marriage, or just random name changes for personal preference. Name changes during transition aren't any different in the amount of time it takes to switch gears.

Doesn't really make it any easier for the trans person, of course, but at least it can help them to know it isn't intentional.