Warren Buffett is famous for his stock-picking ability, but he also puts a substantial amount of Berkshire Hathaway's money in bonds.
Those transactions don't get a lot of attention. Unlike the contents of its stock portfolio that must be filed with the SEC four times a year, Berkshire doesn't have to publicly disclose its debt holdings.
In his letter to shareholders released Saturday, however, Buffett admitted to a money-losing bond buy involving Energy Future Holdings.
(Read more: Buffett's Berkshire slammed by S&P in 2013)