Reading some of the posts in this thread I can't help but wonder if some people don't know how to perform an internet search. Do a Google search for "Man kills bear with knife", or "Man kills mountain lion with knife" and you can read SEVERAL recent and documented incidents of such things, including names, dates, locations, and the types of bear.
Are such events rare? Of course they are. But so are bear and mountain lion attacks.
It really makes me wonder about the people who suggest that it's impossible to successfully fend off or kill a bear or mountain lion with a knife. I guess they don't know how to perform an internet search. Or maybe they just don't want facts to get in the way of acting like an expert.