Don't get annoyed with the store, get annoyed with the people who ruined it for those who honestly made a mistake. Policy is policy, and there's no point getting upset over something the store can't change.

ETA: it's just a good idea to ask about return policy whenever you buy something rather than assuming you'll be able to return it.

I HATE HATE HATE it when people don't read our policy WHICH IS POSTED AT EVERY REGISTER and then get mad at me because I won't take back a jar of paint or a book they bought yesterday.