There are a few tax circumstances where the HMRC tax calculation gives an incorrect amount of tax due.
These vary from year to year, as problems are fixed and new ones arise.
HMRC publishes a set of Exclusions which define these circumstances and give a recommended workaround (commonly, the workaround is to submit a return on paper).
If your tax return fits into one of these categories, you will be alerted when you calculate your tax and the workaround will be suggested. Ftax will not prevent you from submitting the incorrect data, but your return may be rejected by HMRC, or you may be paying the wrong amount of tax if you do.
HMRC have stated:
“Where a personal return cannot be filed online for a reason listed below [an Exclusion] , provided that a paper return is delivered on or before 31st January following the end of the tax year to which the return relates, HMRC will accept that the taxpayer had a reasonable excuse for failing to file a paper return by the normal 31st October deadline. A reasonable excuse claim should accompany the paper return.”