When you want to make an international transaction, the bank runs through the characters of the IBAN across the payment system. By doing so the bank is able to confirm the recipient and sender's accounts. If the accounts are valid the bank goes ahead to process the payment.
If an IBAN is entered incorrectly and there is no account corresponding with that IBAN, the transaction will fail. If a corresponding account exists, the transfer will go through and can only be reversed with the recipient's permission.
On the other hand, most banks use an IBAN checker, also known as an IBAN lookup, to ensure that valid IBANs are free of errors.