laugh it off and pull someones else's pants down and laugh or yell out "Im so sexy I took my pants off for you!!!!" lol hmmm my real answer would be to cry and run away lol double time
Pull them up with a serious face on. :D then walk away or keep doing what i was doing before they fell off, without looking panicked or embarrased.
i heard about this women who was walking where there was huge crowds of people, and her underwear fell right off, onto the ground, then she just walked 'out of them' with aa serious face.