I thought that Krugman had a post about Paul Ryan stating, incorrectly, that Obama had increased the size of the government. And he wondered why conservatives make factual statements that can be easily shown to be wrong. But I cannot find such a post. I can find this one on Rand Paul making a different incorrect factual claim. I am fairly sure I am thinking of something more recent than this post about Rand Paul being confused in 2012.
(By the way, Paul Krugman is wrong about what heterodox economists believe about marginal productivity theory. If he reads this, though, I would rather read his comments about the empirical correlation between increased government size and increased equality.)