The blog Our Valued Customers spotlights comments that artist/comic shop employee Mr. Tim has overheard at work. No doubt these are just the most extreme ones, but looking through them really makes me happy I'm not a customer at his store.
He breaks them down into several categories, like "creeps," "crazies" and "fighters." If you don't want to be too disturbed, I recommend the "Jus' Folks" category.
Update: ComicsAlliance is looking for stories of things you've heard in comic shops ... and are giving away the Jim Lee Icons book to the winner.