According to HMRC, "A very small number of employers have incorrectly been sent an interim penalty warning letter when they have, in fact, submitted their P11D(b) for tax year 2013 to 14. Most of the letters that have been issued inappropriately relate to employers who have changed their trading name recently. If you or your client has received a warning letter and you have successfully filed your P11D(b), you do not need to take any further action."

A "small number" could be anything between 10,000 and 100,000 on the basis of previous penalty letter reporting. This warning is followings at least two other warnings of HMRC sending incorrect penalty letters under PAYE to innocent taxpayers. If the taxpayer made a similar error HMRC would attribute it to carelessness.