How did economists get it so wrong?

In 2009, Paul Krugman wrote an article for The New York Times entitled How Did Economists Get It So Wrong? It is an excellent summary of mainstream economic thinking from Keynes to the present day (albeit from a very US-oriented perspective). It is also very good on the deficiencies of the economics profession in predicting the current economic crisis. I particularly like his analogies about baby-sitting and the price of ketchup.