It's pretty rare, but I've seen it happen once (on a friend's Datsun Roadster)... the floats took on enough fuel internally thru teeny-tiny stress cracks so they couldn't float up high enough to shut the fuel off. You could hear gas sloshing inside them, though. These floats were brass and we managed to coat them in light solder so they wouldn't take on fuel anymore. This was supposed to be a temp fix until he could get new floats, but it worked so well, AFAIK, he never put the new floats in once they finally came over on the slow boat from Japan.