Understanding the scale of a potential taxpayer mistake is fraught with difficulties as HMRC takes a tough line, says Karen EcksteinPractitioners will be aware that the regime for the imposition of penalties for tax omissions changed with effect from 2009. Now, when HMRC considers the level of a penalty to apply to a particular omission, not only is the quantum of the tax omitted taken into account, but the taxpayer’s behaviour determines the category into which it falls. The category of behaviour determines the level of penalty to be imposed.