The Keynesian Endpoint
The Keynesian Endpoint is where governments can no longer borrow anymore money to stimulate or rescue their economies. The term was coined by Anthony Crescenzi at PIMCO in 2010. As a practical matter there are several parts to this process. There is the inability to pay the interest or retire the debt with the current tax revenues (detonation rate), which in turn leads to the inability to borrow any additional money (Keynesian Endpoint). As a practical matter Greece, Japan, Spain, and a few other countries are at or near this point. Based on my limited knowledge of the financial crisis in Iceland, it appears that country reached the Endpoint. The remedy is to either default or re-structure the debt leaving the country in a very weakened position.
The US is beginning to flirt with the maximum it can borrow, although there is considerable debate as to what the maximum amount of borrowings is. I have heard everything from $17T to $25T. The US is currently at $16.5T. The ratio of interest on the national debt to tax revenues is about 9.1%, just in case you were curious about why rates are so low.
Below is diagram of the US situation since 1792 from Addogram.
If you are interested in Crescenzi's book on the Keynesian Endpoint here is a link at Amazon. If you read the book all of sudden "financial repression" and the low interest rates of the 1950s will begin to make sense.