He lost a pile on the airlines and even decided to realize those losses.
He lost a pile on that petroleum company that I forgot the name of, but which crashed and burned recently due to oil prices plunging.
He hesitated a long time with tech stocks and lost a lot of money in the last decade due to opportunity cost. His only good bet in that area was Apple, and admittedly that paid off nicely, but even his big stake doesn't compensate for missing out on basically all other tech stocks. Especially since the size of his Apple investment suggests that he played a significant part in raising the Apple stock value, which means that he probably can't pull that out again without depressing the stock price.
Bottom line: I think Warren Buffett is overrated. He fails, just like anyone else.