72 Pa. Stat. § 5020-505.1
Whenever through mathematical or clerical error an assessment is made more than it should have been, and taxes are paid on such incorrect assessment, the county commissioners, acting as a board of revision of taxes, or the board for the assessment and revision of taxes, upon discovery of such error and correction of the assessment shall so inform the appropriate taxing district or districts, which shall make a refund to the taxpayer or taxpayers for the period of the error or six years, whichever is less, from the date of application for refund or discovery of such error by the board. Reassessment, with or without application by the owner, as a decision of judgment based on the method of assessment by the board, shall not constitute an error under this section.
72 P.S. § 5020-505.1