Actually, Kat, it's "one fell swoop" -- a term that confused me so much when I was a child that I tended to spoonerise it to "one swell foop."   (I guess it's originally a logging term or something like that.)

But I digress.

One malapropism I hear all too often, including in song, is the use of "hoi polloi" to (judging from context) mean "high brow" or "elite." It's obviously a confusion of "hoi polloi" (the "rabble" or "great unwashed") with "hoity-toity."

My brother -- who knows better -- once slipped during a lecture and referred to the movie "Who's Afraid Of Virginia Woolf," in which the two main characters were "always throwing epitaphs at each other."

Then there's the common misspelling that yields: "I'm waiting with baited breath."
(I always wondered what one would use for the "bait.")