After settling into my seat in FC last week, (seat 2B) a harried passenger ran up to my seat and demanded to see my boarding pass citing that I was in his seat. Usually I would tell him to take it up with the FA, but he showed me his OLCI boarding pass with the same seat assignment on it. I stood up and showed him my boarding pass and they matched…2B. He ran off the plane and returned 10 minutes later and sat in 2F. I had been assigned my seat at the 5 day EUA and checked in on line. I know this happens in coach when an upgrade has happened but I have never seen this in FC. I can’t figure out how this could happen especially since we both had OLCI boarding passes. I’m not sure whether this was a one time incident but I will be boarding early in FC for now on so as not to be the one without the seat when the music stops.