I work for a credit card gateway, and Kyralessa is the closest to correct here. some point of sale systems won't allow a transaction to even be posted to an expired card.

However, if it does allow it, then ultimately, how it is handled is at the discretion of the bank that issued the card. Some banks would respond with a decline, and I imagine others would authorize the transaction.

However, if the cardholder calls the back and tells them they did not authorize the charge(s), the issuing back will initiate the charge