Not quite a free upgrade but a mistake by cabin crew. I was flying DUB-LHR-LAX I had a world traveler ticket and got to fly first.
Background story I was checking in at the airport originally had seat 26C for DUB-LHR Aer Lingus. And asked for a window seat and was given 2F. When boarding the LHR-LAX flight I accidentally showed the DUB-LHR boarding pass and when she saw it said seat 2F she said to turn left and I got to fly first class with a world traveler ticket.