This didn't happen too long ago:

http://www.ktvb.com/news/localnews/stories/ktvbn-jun1209-bear_attack_folo.73be7ef6.html

A man seemed to stumble into a no-win situation with a sow and two cubs and a buried elk carcass nearby. I guess he came around a corner and there she and her two cubs were.

Being by himself, the only thing I think this guy might have done wrong was possibly not make noise while he was walking because, everything I've heard about it made this situation seem like it was pure surprise on the part of both parties. But who knows if talking while talking would have even helped either. One of those "what if" situations it doesn't hurt putting yourself in mentally.