Is there such a thing left as a safe investment? Stocks have been massacred, real estate all but wiped out. Each was promoted in its day — as was gold — as safe and secure, appropriate for widows and orphans.

If there is a truly last bastion of safety, it would be, of course, the U.S. Treasury bond, that venerable instrument with the full faith and credit of the United States behind it. Perhaps it is esteemed so highly because we think of it not as an “investment” per se but as an article of faith in Washington and, by extension, the entire country. It is our tax dollars, after all, that stand behind it — the accumulated output of our citizens. And ever since the Wall Street meltdown, as investors have fled from any security carrying a whiff of danger, Treasuries have been in hot demand.

So it is an eye-opener, and rather depressing, to report that even Treasuries bear risk, in particular, the risk that flows from crowd psychology. Last month, in his annual letter to shareholders (of which I am one), Warren Buffett wrote: “When the financial history of this decade is written, it will surely speak of the Internet bubble of the late 1990s and the housing bubble of the early 2000s. But the U.S. Treasury bond bubble of late 2008 may be regarded as almost equally extraordinary.”

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Pretty strong words for an investment that has outperformed stocks over the past 25 years and is widely referred to as “riskless.” Yet according to Buffett and other investors of a cautious bent, “risk free” Treasuries of longer maturities are anything but. None other than China’s prime minister, Wen Jiabao, expressed worry about the safety of China’s big stake in U.S. bonds.