Water under the bridge (over makes no sense unless you're attempting to destroy things, which is the exact opposite of what that phrase is supposed to mean, LOL).
Couldn't care less. I read an article years ago about how when people say "I could care less" the implication is supposed to be "Eh, it's POSSIBLE that I could care less, but it's not really likely." I call bullshit. It's "I couldn't care less."