[citricsquid]

As uptown said, providing valid credit card information creates an authorisation for a merchant to charge to you, if your card expires after that authorisation has already been created you can still be charged and that charge will still be approved by your credit card provider. This is a common problem for people who take out short term loans and then think they can cancel their card to escape being emptied out when due day comes.

Cancelling a card (or letting it expire) prevents new authorisations, it does not cancel previous authorisations.

[ig1]

Also there are automated credit card update services, so if you get a card reissued then vendors can get their card on record updated.