Got onstage for my "3 songs or 12 minutes" open mic spot at The Ark in Ann Arbor, some 15-20 years ago. The MC, about to introduce me, leaned over and whispered in my ear "XYZ!" (For those who don't recognise this pithy little phrase, it's short for "Xamine Your Zipper".)

Instead of having the presence of mind to discreetly turn 'round for a moment and zip up, then face front again, I just reached down and did so (very quickly!) while still facing the audience -- which of course made it very clear that my zipper had been down...

The really silly thing is -- it wasn't all that noticeable from the audience; if I had done nothing in response to the MC's warning, I'd have been better off...

Put the audience in a pretty good mood, though!