A mysterious instruction has been found at the bottom of letters, requesting writers to refrain from writing below a certain line. The request has sparked curiosity and led one individual to investigate further by emailing TVL/BBC in November 2006. According to the provided URL, the reason behind this unusual instruction remains unclear.

Source: http://www.bbctvlicence.com/Please%20do%20not%20write%20below%20the%20line.htm