Mike Shields:

During these tests, Google and the partners shut off all of their programmatic ad inventory for brief periods, say, 10 to 15 minutes, and then scour the ad exchanges to see what’s listed. Google and its partners found thousands if not millions of video and display ad spots still available on multiple ad exchanges, despite no ads actually being for sale at that time, the people said, asking not to be identified because the results haven’t been publicly released.
 
 These include Google’s own AdEx exchange, as well as AppNexus, Oath’s BrightRoll, and PubMatic. Google also discovered fraudsters claiming to be able to sell YouTube ad inventory on various exchanges, one of the people said.
 
 Google’s not alone in these findings. An ad-tech executive from a different company went looking for some spoofed ads on exchanges and said they easily found thousands of such misrepresented ads for sale, and below are the results of another search by the Marketing Science Consulting Group, a company that specializes in researching ad fraud.
 
 Business Insider reached out to all the exchanges mentioned and included their comments below, if they responded.