Brad DeLong is a professor of economics at Cal. Megan McArdle is a hack in the employ of the Atlantic (for some reason).
Megan, meet Professor DeLong.
The real treat here is that McArdle comes by DeLong's blog to defend her stupidity in comments. DeLong, again, tries to clue her in. The fun comes when she finally realizes her simple (yet gargantuan) error and has to finally utter the resounding mea culpa of, "Oops."