5 CFR 831.1402 - Fault.
A recipient of an overpayment is without fault if he/she performed no act of commission or omission which resulted in the overpayment. The fact that the Office of Personnel Management may have been at fault in initiating an overpayment will not necessarily relieve the individual from liability.
(a) Considerations. Pertinent considerations in finding fault are -
(1) Whether payment resulted from the individual's incorrect but not necessarily fraudulent statement, which he/she should have known to be incorrect;
(2) Whether payment resulted from the individual's failure to disclose material facts in his/her possession which he/she should have known to be material; or
(3) Whether he/she accepted a payment which he/she knew or should have known to be erroneous.
(b) Mitigation factors. The individual's age, physical and mental condition or the nature of the information supplied to him/her by OPM or a Federal agency may mitigate against finding fault if one or more contributed to his/her submission of an incorrect statement, a statement which did not disclose material facts in his/her possession, or his/her acceptance of an erroneous overpayment.