This would have been 14 years ago or so. I was in my 20s and it was my first domestic flight in the U.S. - a westbound Delta transcon. Also, my first Delta flight period. I got an operational upgrade to 1st in spite of not having status or anything. I simply arrived early at the counter (like 2-3 hours before the flight) to request my boarding pass and the staff person there said "good news, you're now in first class, here's your boarding pass".
At the time I thought I thought I was lucky - but I had no way of realizing just how lucky it was to be upgraded as a very infrequent flyer up to that point without any notion of what a frequent flyer program is. I still recall being very impressed by the food. It was an evening flight and the dinner was enchiladas with chocolate cheesecake for dessert. Both were very tasty and certainly the best thing I had ever eaten on an airplane up to that point.
I think it actually played a pretty significant part in getting me interested in premium cabin travel and gave me a good opinion of Delta as an airline at the time (and even to this day I instinctively prefer them over AA and UA).