35 ILCS 200/14-10
If the county assessor in counties with 3,000,000 or more inhabitants, at any time prior to the time the board of appeals (until the first Monday in December 1998 and the board of review beginning the first Monday in December 1998 and thereafter) is required to complete its work and adjourn under Section 16-150, certifies to the board that there is a mistake or error (other than a mistake or error of judgment) in the valuation or assessment of any property, or in the entry of any assessment in the assessment books, the county assessor shall set forth the nature and cause of the mistake or error. The board of appeals (until the first Monday in December 1998 and the board of review beginning the first Monday in December 1998 and thereafter) shall give the person affected by the assessment notice an opportunity to be heard. If the board of appeals (until the first Monday in December 1998 and the board of review beginning the first Monday in December 1998 and thereafter) is satisfied that a mistake or error has occurred, the majority of the members shall endorse it by signing the certificate and shall order the assessor to correct the mistake or error.
35 ILCS 200/14-10