One phrase that has two opposing camps arguing its origins is "Cheap at half the price." Taken literally, it makes little sense - if something were half price, then it's reasonable to assume that it would be cheap. One camp argues that the meaning is ironic; that what is intended is, in fact, that the item in question is ludicrously expensive. The other side argues that the expression originates from medieval times, "cheap" (or "cheep") meaning not price but goods. Thus, goods at half price. One phrase, two opposing meanings. Take your pick!