My mother was very guilty of this, and her assumptions were usually extremely bigoted.

Every time I mentioned someone by first name she demanded to know their last. Then she would run a commentary on the ethnic/religious origin and her assumptions about the name. This was so frustrating and I would inform her that such thinking was prejudicial and not appropriate. I would also point out to her that her own surname was Scandinavian yet she herself was actually Scottish. "Mom," I would say, "last names mean nothing." But she would shake her head and go on about her extreme prejudicial assumptions and coinciding paranoia. I am not kidding here. This whole verbal transaction would happen every time I mentioned a name.