I think I've mentioned this story before, but it does fit....
I'd been at a professional conference, at which I was wearing my best "dress clothes". However, it was over, and I'd headed out to the airport, the first thing I did was go to the washroom and change into jeans and t-shirt for comfort on the trip back.
As I was heading towards the gate, I passed another conference attendee, waiting at his gate. In a very loud voice, he called across the concourse, "Oh, THERE YOU ARE, TWIK! I'VE NEVER SEEN YOU WITH YOUR PANTS ON BEFORE!"
The funniest part was the stranger beside him who manfully struggled for 30 seconds before dissolving into giggles.