It happens all the time in fandom, and even more in the SCA and similar organisations. Lots of people look blank when they use my real name, and then realise "Oh, you meanKeris, why didn't you say so?" I've come across it outside as well, even at work where I often can't remember the surname of a cow-orker because we hardly ever use it. And then there's places like China, where people have a Chinese name and then an 'English' one for use with foreigners (often with an anglicised form of their family name as a 'surname').

And of course a very large number of celebrities don't use their birth names, and in many cases their birth names have been hidden from all except family...