I'm not sure this was my first trip in first class, but it was my first international and the earliest one I remember.

Connecting in Zurich going from Milan to Boston, I spent a bit too much time shopping and got to my gate late. The rigid Swissair gate agent scowled and said "We have given your seat to another passenger." When I apologized and asked if there was any way I could get on the plane, she said, meaning every word of the first five with the utmost seriousness, "Yes, but I am sorry. You will have to fly in first class."

I managed to indicate that this would be acceptable under the circumstances, took my boarding pass and got on.

Like swag's boss, mine boarded and saw me as the FAs were peeling me grapes and pouring me champagne or whatever they did. Then he walked through first and business on the way back to his coach seat. (He traveled more than I did, but not enough to earn upgrades in those days of few FF programs.) He never asked how I got there - he wasn't much of a communicator. I didn't stay with that company long.