I have done this often. The results are amazing if you succeed.
You casually walk up to some guy you know who has more confidence than you feel he deserves. You look around casually as if thinking about something (which you aren't) you do this for a little bit while the guy is absorbed in whatever. Than you react as if surprised as if you noticed something about his fly in his pants being open. (Don't overdo the act!)Than in a way that is best for you.....NOT SOMEBODY ELSE.... tell him that his fly is CLOSED. (in the same way you would if you were telling him that his fly IS actually OPEN. If distracted enough he WILL attempt to CLOSE his fly only to be dumbfounded to see that it is closed. So now just relax and watch as he deals with this perplexing issue until he realises that YOU DID TELL HIM THAT "HIS FLY IS CLOSED" Than wait to see how he deals with this info!
I have seen such confident guys become speachless and flustered so fast that I can truely say that it is COOL to watch and observe. But I always go after the guys who have some superior edge over me, a boss for example or a "class clown" who always gets the last word, etcetra.
Thanks for reading my post!