One possible consequence of defaulting on the national debt? Well, according to the director of the Congressional Budget office, it could ... increase the deficit:

Elmendorf noted that one of the potential consequences of even a brief period of default would be higher federal debt, triggered by a spike in interest rates and, thus, higher interest payments on federally issued debt.

“If Treasury rates moved up by just 10 basis points over the next decade, that would add $130 billion to interest payments over the decade,” Elmendorf said. A basis point is one-one hundreth of one percent. Thus, according to Elmendorf, each 0.1 percent increase in interest rates on U.S. Treasuries would amount to a significant increase in U.S. debt.