If we're allowed to bring in phrases, I'd like to nominate "could care less". The correct phrase is "couldn't care less", of course, because the sentiment is that one hasn't even the slightest interest or concern about something.
How about, "pure as the wind-driven snow"? New fallen snow can be rather pure but when the wind drives it over the landscape, as one of my characters said, "Wind driven snow isn’t pure, it gets all mixed up with dirt and leaf bits and dead grass and dog poop and gosh knows what all else." Some people use "pure as the wind-driven snow" sarcastically but it's become common to hear it used synonymously with "pure as new-fallen snow".