I could care less - This expression actually means the opposite of what it seems to say.
Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo - A grammatically correct sentence that's just ridiculous.
Literally - Now used to mean both literally and figuratively, which is literally the worst.
Phrasal verbs - Like "put up with" meaning tolerate, because why use one word when three will do?
Homonyms - Words like "bat" (animal) and "bat" (sports equipment) that make English learners want to bat something.