Buffett was reasonably diversified even in his partnership days. I think some people take the concentration idea too far. Mistakes happen but no one mistake should kill you.
Here are the criteria Buffett laid out in his partnership letter dated January 20, 1966.
"We are obviously only going to go to 40% in very rare situations - this rarity, of course, is what makes it necessary that we concentrate so heavily .when we see such an opportunity. We probably have had only five or six situations in the nine-year history of the Partnership where we have exceeded 25%.."
Page 9 (Diversification) http://www.rbcpa.com/WEB_letters/1966.01.20.pdf