53 Pa. C.S. § 8816
(a) Correction.--If, through mathematical or clerical error, an assessment is higher than it should have been and taxes are paid on such incorrect assessment, the county assessment office, upon discovery of the 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 the error by the board. Reassessment, with or without application by the owner, as a decision of judgment based on the method of assessment, shall not constitute an error under this section.
(b) Increases.--Nothing in this section shall be construed as prohibiting an assessment office from increasing an assessment for the current taxable year upon the discovery of a clerical or mathematical error.
53 Pa.C.S. § 8816