The treasury bond bubble will burst

This excessive debt level of the U.S. Federal government insures that Treasury bonds will never be repaid in real terms. The market is aware of this situation—the bond market is aware that Treasuries are in a bubble, floating on nothing but air. Therefore, when—not if—the bubble in Treasury bonds finally bursts, there will be a run on comodities, most likely, which will start the hyperinflationary phase of the current Global Depression. From here, the endgame of the U.S. economy.
--http://www.zerohedge.com/article/guest-post-prosecution%E2%80%99s-case-against-alan-greenspan

Most people see US Treasury bonds as the ultimate safe haven, so this is kind of hard to imagine, but I will try to explain. There is an inverse relationship between bond prices and interest rates - when interest rates go down, bond prices go up and when interest rates go up, bond prices will go down. Guess what - interest rates will go up at some point, which may be years away. When this happens, investors will flee bonds. US bonds are a stupid investment.