In the spring of 1979, Congress was in the midst of a similarly heated debate about raising the debt ceiling, Legislators eventually reached a last-minute deal to raise the debt ceiling and (they thought) save the day, but something went wrong. The Treasury didn’t redeem $120 million worth of securities that matured in April and May.
In other words, the U.S. Treasury defaulted on its securities even though Congress settled the debt-ceiling issue. What happened? It’s not totally clear.
Here's the short answer: When all else fails, blame the computers. Read a more thorough explanation at mental_floss. Link