Technical errors makes customerw pay

So I get a sms from Nedbank saying that the serif is on his way to my place of work??? No I know that I I'm not in debt with anyone so what's the problem. Apparently in November of last year nedbank had a technical error which resulted in one of my accounts being credited twice and that I now owe nedbank money???? My question is this. Since when is it the fault of the customer when the service makes a mistake??? FIY I also got a sms in April with regards to a double payment. When I called to find out what was happening the assistant on the other line confirmed that this problem was resolved now almost 8 Months later I'm being held responsible for payment. With customer service like this no wonder people want to close their accounts and move to other banks. Thank you nedbank for confirming that I need to do this also