One time, I had a seat in first class, confirmed. However, I boarded last and just told my wife 'Just take those seats' in the bulkhead. Apparently the lady across the row heard me, balked at the flight attendants, who then asked my wife and I for our boarding passes.

Of course since we were meant to be in first, it didn't matter. I would never sit in first unless I was meant to be there. I think the lady felt very embarrassed since she tried to make nice talk the whole flight...

Personally I don't think it's right for someone to take a first class seat if they don't belong there, but I also don't think it's right for another passenger to turn them in. It should be the FA's duty to check the manifest and reconcile. It's only a few rows of seat -- shouldn't be so difficult!