Default

Technically the US has defaulted before...on accident, and it wasn't pretty:

Terry Zivney and Richard Marcus describe the default in The Financial Review (sorry, I can’t find an ungated version):

Investors in T-bills maturing April 26, 1979 were told that the U.S. Treasury could not make its payments on maturing securities to individual investors. The Treasury was also late in redeeming T-bills which become due on May 3 and May 10, 1979. The Treasury blamed this delay on an unprecedented volume of participation by small investors, on failure of Congress to act in a timely fashion on the debt ceiling legislation in April, and on an unanticipated failure of word processing equipment used to prepare check schedules.