From a recent report by Social Security's Office of Inspector General:
SSA needs to improve its controls to ensure it notifies disabled beneficiaries when they are eligible for higher retirement benefits. Based on our random sample, we estimate that 17,137 disabled beneficiaries were eligible for higher retirement benefits totaling approximately $105.6 million. This occurred because SSA employees did not inform disabled beneficiaries when they were eligible for the higher retirement benefits or document the reasons beneficiaries did not elect higher retirement benefits, as required.