One of the greatest areas of misunderstanding is doubling. Is it penalty? Is it take out? Is it negative? Recently I heard about a pair who were playing exclusion blackwood (or at least one of them was) and ended up playing 5♣ (undoubled) in a 0-0 fit. True story.  You need trumps to double a game (unless they have obviously screwed up) but it will usually cost a trick.