For example if your Facebook password is "qWerty" then you can login with another password "QwERTY"
i.e. your original password with case reversed.
After reading an article in ZDnet news I have tried this on my Facebook account and its working. In fact this is not a loophole in Facebook but a feature by the Facebook design team.
Facebook has noticed that a lot of people facing trouble while logging into their Facebook account because of CAPS-LOCK key. So Facebook introduced this multiple password feature so that a user can log in to his account even CAPS-LOCK is on.
