Most overpayments happen because customers don't tell us about changes straight away. Sometimes overpayments happen because it takes us a little while to process the change. Sometimes we can make mistakes at our end.
If we make a mistake this type of error can only be an official error overpayment if the mistake is down to us and what we, the Department for Work and Pensions (DWP) or Her Majesty's Revenue and Customs have done.
It's your responsibility to tell us when your circumstances change. If you haven't told us and an overpayment has been made then it's not an official error.