40 ILCS 5/2-155.1
If the System mistakenly sets any benefit at an incorrect amount, it shall recalculate the benefit as soon as may be practicable after the mistake is discovered.
If the benefit was mistakenly set too low, the System shall make a lump sum payment to the recipient of an amount equal to the difference between the benefits that should have been paid and those actually paid.
If the benefit was mistakenly set too high, the System may recover the amount overpaid from the recipient thereof, either directly or by deducting such amount from the remaining benefits payable to the recipient. However, if (1) the amount of the benefit was mistakenly set too high, and (2) the error was undiscovered for 3 years or longer, and (3) the error was not the result of incorrect information supplied by the affected member or beneficiary, then upon discovery of the mistake the benefit shall be adjusted to the correct level, but the recipient of the benefit need not repay to the System the excess amounts received in error.
This Section applies to all mistakes in benefit calculations that occur before, on, or after the effective date of this amendatory Act of the 98th General Assembly.
40 ILCS 5/2-155.1