Let us make a little list:

Tyler Cowen points out that when it comes to discussing Milton Friedman's analysis of the Great Depression, Paul Krugman gets things wrong.
Both Tyler Cowen and Donald Boudreaux point out that Krugman gets Hayek wrong.
Raghuram Rajan points out that Krugman gets Carmen Reinhart and Kenneth Rogoff wrong, though I am not sure that Krugman actually engaged in a classic ad hominem. It seems that he was just needlessly insulting and made ludicrous charges against Reinhart and Rogoff, which is par for the course for Krugman when it comes to dealing with people he views as enemies.

That's a lot of things to get wrong in a short period of time. Fortunately, as ever, Krugman proves himself equal to the task.