Jonathan Owen, over at Arrant Pedantry, on twelve common mistakes made by people who write about grammatical mistakes.
Fair warning, for those who want it: Like most scholars of linguistics, he’s a descriptivist, not a prescriptivist. (As the header of this post suggests, so am I.) If descriptive grammar is the sort of thing that makes your milk of human kindness go all sour and curdled, you probably don’t want to go there.