HMRC has acknowledged a mathematical error in the pension tax system, affecting approximately 1.7 million state pension recipients. The issue is attributed to the self-assessment system incorrectly pre-filling state pension income at the new rate for 52 weeks, instead of the correct correct week at the old rate and 51 weeks at the new rate. As a result, HMRC is reviewing potential reimbursements and advising affected pensioners to verify their state pension amounts and PAYE coding notices for any necessary past-due adjustments.