An employer is legally entitled to recover any overpayment of wages, either during the currency of the contract of employment, as well as after the employee's contract has come to an end.
You have the right to appeal an overpayment determination. You must submit your appeal in writing within 30 days of the mailing date on the Notice of Overpayment (DE 1444). You can still submit an appeal after the 30-day deadline, but you must provide the reasons why you missed the appeal deadline.
The general rule is that if an employer has overpaid an employee, the overpayment of wages should be repaid even if the mistake was the employer's.