HMRC are intended to scrap it’s long standing concession known as ESC A19 which allowed it to cancel tax bills that had risen because of error by it’s staff. This was to be replaced with a more modern, revamped version of the rules, but the draft version was met with complaints from tax experts saying it was too stingy.
Fortunately, the Taxman seemed to listen on this occasion to the protests and confirmed that the concession will stay.
As HMRC is keeping it’s concession ESC A19 you can ask for this to be applied to cancel a tax bill which may have risen because of their error, fo e.g where you told them about other income to include in your tax code but the information was ignored or not amended.