Berkshire Hathaway CEO Warren Buffett, the legendary exponent of "value investing," was once asked to suggest the best time to sell shares -- and famously replied, "Never."

[Buffett and Federal Reserve Chairman Alan Greenspan agreed to disagree about the effect that so-called "derivative securities" would have on financial markets. Greenspan said they had reduced risk. Buffett's verdict? In his letter to shareholders in 2003, Buffett called them "weapons of mass destruction."]