I had an interesting situation the other week. The first leg of my trip was SDF to ATL on a MD-88. I had been upgraded to FC with seat 4A assigned. When I boarded there was a lady sitting in 4A already. Thinking she was in one of the other seats I spoke to her and told her I was in 4A and could she check her boarding pass. She got "huffy" with me and pulled out her BP and informed me "she was assigned 4A and I must be wrong". Not wanting to start any stuff I called the FA over, who asked for her BP. The FA looked at the pass, looked at my BP on my phone and stated there was a mix-up as we were both assigned 4A. At this time I actually saw the woman's boarding pass and saw what the problem was. I informed the FA that there was not a screw up but I had 4A and the woman actually had 34A. Seems the FA and the woman in my seat had looked at the departure gate (4A) and thought it was her seat. She was not happy to move to the ""back of the bus" in her assigned seat.