40 ILCS 5/17-151.1
If the Fund mistakenly sets any benefit at an incorrect amount, the Fund shall recalculate the benefit as soon as may be practicable after the mistake is discovered. The Fund shall provide the recipient, or the survivor or beneficiary of the recipient, as the case may be, with at least 60 days' notice of the corrected amount.
If the benefit was mistakenly set too low, the Fund shall make a lump sum payment to the recipient, or the survivor or beneficiary of the recipient, as the case may be, of an amount equal to the difference between the benefits that should have been paid and those actually paid, plus interest at the rate of 3% from the date the unpaid amounts accrued to the date of payment.
If the benefit was mistakenly set too high, the Fund may recover the amount overpaid from the recipient, or the survivor or beneficiary of the recipient, as the case may be, plus interest at 3% from the date of overpayment to the date of recovery. The recipient, or the survivor or beneficiary of the recipient, as the case may be, may elect to repay the sum owed either directly by a lump sum payment, in agreed-upon monthly payments over a period not to exceed 5 years, or through an actuarial equivalent reduction of the corrected benefit. However, if (1) the amount of the benefit was mistakenly set too high, (2) the error was undiscovered for 3 years or longer from the date of the first mistaken benefit payment, and (3) the error was not the result of incorrect information supplied by the affected member, then upon discovery of the mistake the benefit shall be adjusted to the correct level, but the recipient of the benefit shall not be required to repay to the Fund the excess amounts received in error.
40 ILCS 5/17-151.1