Long ago - 1982 I had purchased a ticket which turned out to be an award ticket on CO. Standing in the business line - that's what I had purchased - the agent in First Class called me over. Turned out my ticket was in First Class on the 747 when CO had them. Great flight and a great introduction to New York with that great view landing at Newark.
My other memorable flight was a Christmas Day on UA from LHR-SFO on the old 747-200. I got an upgrade - and was the only passenger in First. The crew enquired if I minded if the Captains wife joined me (dare anyone to say NO), and we had the most wonderful flight all the way! I think I put on 20 lbs!