The BBC was able to pull this off thanks to the fact that Twitter collects a variety of data on its users. Like a lot of other websites and services, it packages this information in such a way that advertisers can micro-target groups of individuals. Typically, companies might specify groups such as "amateur photographers" or "parents of teenagers" when they use this feature. However, the BBC was able to use those same tools to target people who had shown an interest in far-right ideologies.

It shouldn't have been able to do this. One of Twitter's policies on keyword targeting prohibits advertisers from using terms that fall under "sensitive" categories such as "political affiliation or beliefs," "racial or ethnic origin," and "sex life." Twitter blamed an unspecified error for the fact the BBC was able to bypass its safeguards.

"We're very sorry this happened and as soon as we were made aware of the issue, we rectified it," said a Twitter spokesperson. "[Our] preventative measures include banning certain sensitive or discriminatory terms, which we update on a continuous basis. In this instance, some of these terms were permitted for targeting purposes. This was an error."

They went on to add, "We continue to enforce our ads policies, including restricting the promotion of content in a wide range of areas, including inappropriate content targeting minors."

While Twitter made an honest mistake in this instance, the error highlights just how easily micro-targeting could be abused to take advantage of certain political groups. It's for that reason that companies like Google have stopped allowing advertisers to target people based on their political leanings or public voting records. Facebook has also reportedly considered restricting the practice. For its part, Twitter doesn't allow the majority of political ads at all.