http://nymag.com/daily/intelligencer/2013/04/grad-student-who-shook-global-austerity-movement.html wrote:
Herndon pulled up an Excel spreadsheet containing Reinhart's data and quickly spotted something that looked odd.
"I clicked on cell L51, and saw that they had only averaged rows 30 through 44, instead of rows 30 through 49."
What Herndon had discovered was that by making a sloppy computing error, Reinhart and Rogoff had forgotten to include a critical piece of data about countries with high debt-to-GDP ratios that would have affected their overall calculations. They had also excluded data from Canada, New Zealand, and Australia — all countries that experienced solid growth during periods of high debt and would thus undercut their thesis that high debt forestalls growth.
"I clicked on cell L51, and saw that they had only averaged rows 30 through 44, instead of rows 30 through 49."
What Herndon had discovered was that by making a sloppy computing error, Reinhart and Rogoff had forgotten to include a critical piece of data about countries with high debt-to-GDP ratios that would have affected their overall calculations. They had also excluded data from Canada, New Zealand, and Australia — all countries that experienced solid growth during periods of high debt and would thus undercut their thesis that high debt forestalls growth.
Economic studies are beyond my realm of interest, but this seems like a fairly critical (and yet astonishingly simple) "error".