It is possible that your customer makes a claim that payment was credited to you whereas you have not received it.
This can be due to network error/bank switch failure, such that the customer received a failed/unsuccessful notification whereas you are credited for the transaction. Also, due to network timeout, there are scenarios where a customer might be debited even though they have not completed the transaction cycle.
At such times, the first thing to do is to login to your account and review your transaction history using the email or transaction ID provided by your customer. You can then proceed to complete the order on behalf of your customer.