You know, for either case, either a guy's fly open or a woman's skirt in her pantyhose... whether or not I tell them depends on who they are. If they are my friend, are generally nice to me, or I don't know them too well, I'll politely tell them to fix themselves, b/c you know, I want to know these sorts of things myself. But if they're mean to me, a jerk, or something of that nature, heck, they are on their own! Serves them right! I have told complete strangers about this kind of problem, it's a bit embarrassing, but you know, they are more embarrassed than you, and generally pretty thankful that you stopped to tell them.