Q: For the first time in my life, I am paying tax on my pension. It is not a huge amount, but I am wondering if HMRC made a mistake, as my tax code has changed?
A: Without knowing your personal circumstances and the amounts involved, it is hard to say if HMRC has made an error. It is not infallible, though. You mention that you tax code has changed, so you should have received correspondence from HMRC explaining why this has occurred. If not, you should get in touch with them or you can, of course, engage with us to deal with HMRC on your behalf.
Given that the annual tax-free personal allowance of £12,570 is still frozen, it is probable that any increase to your pension income (state and private) has now taken you over the threshold. Many do not realise that pensions are taxable, and so you are likely now paying income tax on the proportion that is above £12,570.