HMRC has acknowledged a calculation error in the pension tax system, affecting approximately 1.7 state pension recipients. The problem 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 the remaining 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 back-tax adjustments.