Tim Harford tells the story of the student who uncovered a mistake in a famous economic paper that has been used to make the case for austerity cuts. In 2010, two Harvard economists published an academic study, which showed that when government debt rises above 90% of annual economic output, growth falls significantly. As politicians tried to find answers to the global economic crisis, “Growth in a Time of Debt” by Carmen Reinhart and Kenneth Rogoff was cited by some of the key figures making the case for tough debt-cutting measures in the US and Europe. But, in the course of a class project, student Thomas Herndon and his professors say they have found problems with the Reinhart-Rogoff findings. What does this mean for austerity economics?