At a gig, only once. Pretty sure it was a bad string, unwrapped at the ball end, only string that ever broke on that guitar. I'm really rough on strings, too, but I change them religiously and always carry at least two guitars. Not necessarily as a backup (though backups are a really good thing to have and not need), but just to change it up some.
When I first started, I did all the normal beginner things like wrapping them over themselves on the last wind, not wrapping enough, general setup issues like saddle burrs or incorrectly cut nut slots. I learned, though... I think I once went through 3 or 4 sets just to get one high E on right! Now, I get a bad string every few years or so, but I can't complain.
Had a friend who worked in produce at a grocery store. He could just look at a guitar and corrode the strings, rust out the bridge. Could've been just different body chemistry, but I always wondered if it had something to do with the acid from the fruits and vegetables he handled. Not a clue, but I don't think I ever remember him breaking a string on stage. He changed his strings all the time because he had to, they just became unplayable too quickly if he didn't.