Since the presidential election, hundreds of companies have decided to block their advertisements from running on Breitbart News, the alt-right website closely tied to President Trump's administration. But several of those brands, from the Nordstrom department store chain to small start-ups, have appeared on the site anyway, another example of how little control companies often have over where their ads are seen online.

Nordstrom, which said it moved to prevent its ads from running on Breitbart several months ago, was on the site as recently as two weeks ago, puzzling employees and others online, who were quick to take screenshots and question the company on Twitter. Other advertisers, including BMW of North America and Scribie, a transcription service, also appeared on Breitbart after blocking their ads from the site, a practice known as blacklisting.