Jessica Toonkel, writing for Reuters:
Some advertisers are working overtime to scrub their spots from websites including Breitbart News, an unintended consequence of the automated ad buying systems that are meant to lower costs and allow for more targeted advertising.
Those trying to keep their ads off certain websites are finding they must take steps to verify the spots they bid for are where ads actually appear and that there are no third parties involved that can result in ads winding up in unintended places.
Another consequence of just throwing your ads up everywhere and praying for clicks: you don’t know where your brand ends up being shown, and what it’s next to.