Scott introduced the concept of a motte and bailey doctrine on Slate Star Codex, in an article called Social Justice and Words Words Words or something like that. I don't think he said anything that was wrong in that post (about that concept), but it appears that a lot of readers who hadn't heard about M&BDs before misunderstood it to be about a debate tactic/fallacy. So on SSC and to some extent on LW 'motte and bailey' is often used with the meaning 'bait and switch'.