From a recent report by Social Security's Office of Inspector General (OIG):
SSA [Social Security Administration] needs to improve its controls to ensure WEP [Windfall Elimination Provision] is timely and accurately applied for Federal pensions. We determined WEP should have been applied to 14 of the 250 beneficiaries sampled . We did not identify any beneficiaries for whom GPO should have been applied. Although SSA was aware these 14 beneficiaries had a government pension, the Agency did not reduce their benefit payments for WEP. The 14 beneficiaries received about $372,000 in overpayments. Based on our sample results , we estimate SSA overpaid about $129 million in Old-Age, Survivors and Disability Insurance benefits to about 4,900 beneficiaries.