As soon as the customer has arrived on his bank's page, a 30 minute time-out starts at the bank.
At the same time, in your Ingenico ePayments account a similar transaction time-out for the payment page applies. By default, this time-out is much longer than 30 minutes, in which case you have nothing to worry about.
If however you have had the transaction time-out in your Ingenico ePayments account changed to a number below 30 minutes, it's possible that a customer finishes a payment on his bank's side, while the transaction on Ingenico ePayments's side has already expired.
Although this will not affect the customer's payment, he will get a message that his payment was refused, and the transaction in your Ingenico ePayments account will be in status '2 - Authorisation denied'.