One of two things could happen. If the transaction is of a type that the bank is not able to return, a debit card purchase, for example, it might force the account to reopen, post the transaction, impose an overdraftfee and start trying to collect.
If the bank is able to return the electronic entry and does, it will get bounced back to the originator or merchant involved. If you owe the money, that person will have to contact you to get paid.
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