Ill. Admin. Code tit. 50 § 4450.40
All Article 3 and Article 4 pension funds shall institute and maintain policies and procedures to discover and adjust any benefit that is set at an incorrect amount. Any pension fund that discovers and adjusts, if otherwise allowable by law, any benefit pursuant to this Part, shall notify the Public Pension Division of the Department, by mail, of that adjustment of benefit within 45 days after making the lump sum payment or adjustment. This notice shall contain the lump sum benefit payment amount, the interest portion of that amount, the dates and amounts of the incorrect benefits, the adjusted and corrected benefit amounts, the nature and description of the mistake, and the name of the recipient and pension fund.
Ill. Admin. Code tit. 50, § 4450.40