It sounds like you purchased some coin before a recent deposit cleared the banking system, and the deposit ultimately failed.
Is that even possible? I thought exchanges would wait until there is a clear confirmation about the transfer. Sounds like it's their mistake but the user needs to pay for it. It sounds fishy if that's true.
If I am not mistaken, Coinbase allows "instant" purchases when a credit/debit card is attached to a users' account.