US Government Default
"In early 1988 I attended an international investment conference in Fort Lauderdale, Florida. Many of the delegates were shocked when one of America's top investment advisors outlined a depression scenario that put such a strain on US government finances that Washington would be forced to declare a moratorium on interest payments on bonds and Treasury bills. The odds are heavily against such draconian action, of course. But the point is that even such supposedly risk-free securities as American Treasury bonds could conceivably be hit if conditions were bad enough."
Gordon Pape, "Low-Risk Investing"