When people say “I could care less”. Most people do this, but if they would think about it for a second the correct expression is “I couldn’t care less”.
I might venture a little bit of defence of the “I could care less” version. I agree, “couldn’t care less” makes more obvious sense, and it’s the form which I use. However, the “could” usage is, I feel, not completely nonsensical.
“I couldn’t care less”: I have no interest or concern whatever about this matter – there is no way I could care any less about it, than I do.
“I could care less”: I have almost no interest or concern about this matter – perhaps I could indeed care even less about it, but I’d have to try hard, to perform that feat.
The discussion board Wordreference.com (English only), which focuses on points of English usage, once had a five-page thread about this issue. Consensus was, that the “could care less” version is the one which very many people use, rightly or wrongly; and that, as above, one can claim that it sort-of makes sense.