STFU

One of my pet peeves that's been coming up lately is people who don't know when to keep quiet, or at least not shout as loud as possible. My first clear memory of this problem was in high school when I made some quiet joke to someone about someone walking by. The ditz replied very loudly "I don't get it!" and despite my attempt to quiet her down, when she got the joke she happily cried, "Oh I get it! You're saying he's...!" I don't think there was anyway for the target of the joke to have not heard it. And I'm positive the two weren't friends, so there was no reason for her to do it deliberately, she was just a moron.
I find myself encountering this again at work; when I notice an attractive female customer, I honor my manly duty and make sure the nearest male coworker has the opportunity to check her out as well. But some people don't get the rules here. One coworker, a thuggish Mexican, has a complete inability to control his volume, so that when I discreetly direct his attention to a hot girl he'll loudly (perhaps vulgarly) agree with me so that the girl, indeed most of the restaurant, knows what we were talking about. Needless to say, I've stopped pointing them out to him.
So if you're going to be a loudmouth, you'd better be prepared to miss out on what everyone else notices around them.