I haven't boarded the wrong plane, but I did line up at the wrong gate and watched my plane depart without me. I was connecting through ORD and was supposed to go to gate 8. I was walking down the hall - sign says 2, people gathered outside... 50 feet further, sign says 4, people gathered... 50 feet later, sign 6, people gathered.... 50 feet later, people gathered.

I assumed I was at 8. It wasn't. It was absolutely, unequivocally my fault, and a total brain fart on my part. The gate agent said something about "delay," everyone groaned... I returned to the club.

It was 15 minutes later when I went to check flight status and I saw my original flight switch to "departed" that I clued in something had gone... wrong.

Thankfully the airline just laughed and scheduled me on the next flight. Cost me an extra night in a hotel rooom, and a confirmed upgrade. I figure it was a cheap life lesson.