i've done this twice in my life. The first time we were students, but new, didn't really know each other, and we were lying with our legs up the wall (hence the air going in...) and then mine did a huge FF. The others laughed and thoughed i'd dropped a cheeky fart and i wasn't sure which i preferred - somehow the idea that i'd done that on purpose was more embarrassing than the concept of an involuntary FF. I was known as Grinning F (after the contemporaneous hit single Axel F) for ages...
It then happened in front of my boss, about 15 years ago (no legs up walls this time...) i have NO IDEA how or why it happened, and it never did again. i just sidled backwards out of the room and it was never mentioned again. she was exceptionally strange, though.