If you assume something it makes me smile a little on the inside because people's facial expressions are amusing and interesting when they find out they've made a wrong assumption. When people assume the wrong thing about me, I sometimes just let it go. When I do correct the person I do so with a smile to try to nonverbally communicate that I'm not offended (it takes a lot to offend me), but I think I just make the person feel more awkward.
I once went to a concert with a friend. It happened to be a country music concert and my friend happened to be white. There was a college student, also white, there taking a visitor survey for the city. The assumption she made was that my friend was the one there for the concert and I was just tagging along because friends do that type of thing for each other. Where her assumption went wrong is which friend was tagging along with which friend. I'm the country music fan and talked my friend into coming so I didn't have to be alone.
I guess it would be better to say I dragged my friend to the concert. My friend couldn't even remember who we were there to see. Which is what I told the young lady. She responded with a simple "oh" and tried unsuccessfully to mask all her thoughts and emotions: shock (A black woman with locs is the one into country music?!), introspective debate (Did I just racial profile someone? Nah, I'm not that kind of person.) and embarrassment.
I collect moments like this in my memory vault and pull them out when I'm writing. Sometime my character's experience is the same as my, but to make it fun sometimes it's from the other person's perspective. Realistic moments like this in writing is what makes the writing relatable. Everyone has been in some situation like this.