Ad fraud follows the money, and this past weekend, the money was on the fight between Floyd Mayweather Jr. and Conor McGregor.

Such fights that are only legally accessible via pay-per-view are heavy targets for illegal watching. Pirated events are problematic for ad buyers because the illegal streaming sites users flock to are hotbeds for ad fraud. On Saturday nights that have big fighting or wrestling PPVs, 50 percent more ad fraud occurs than on an ordinary Saturday night, according to ad measurement and verification firm DoubleVerify.

Since the piracy from big PPVs can contribute to wasted ad spend, buyers switch from blacklists to whitelists, monitor individual domains closely and avoid cheap inventory to reduce their risk of ad fraud during the event.

One way fraudsters buy traffic from illegal streaming sites is by working with an ad network that embeds a single-pixel frame onto the streaming site, said Ken Van Every, senior business development manager at programmatic buying platform DataXu. Since the pixel isn’t visible on the streaming service, it allows the fraud site buying traffic to get paid for thousands of impressions the user never sees.

What makes these sites tough to detect is they blend in seemingly organic traffic with their paid traffic. For example, the fraud sites will set up Twitter accounts that automatically tweet links associated with the most popular hashtags of the day, said Marc Goldberg, CEO of anti-ad fraud firm Trust Metrics. Some people following the hashtags click on the links out of curiosity. Even if a link takes them to a junk site and they click out of it, the user unwittingly just contributed human traffic to the fraud site, and if the fraud site gets enough human traffic, it might avoid getting flagged by verification services.

Stephani Estes, vp of digital strategy at ad agency Cramer-Krasselt, said she works with a verification firm to monitor domain spoofing — where unscrupulous publishers and ad tech companies obscure the nature of their traffic to resemble legitimate websites — to mitigate fraudsters’ targeting of popular events.

Other ad buyers said they move ad spend from open exchanges to private marketplaces and adopt whitelists (which limit spend to fewer websites than blacklists) when they’re concerned about a specific event exposing them to fraud. Matt McLaughlin, COO of DoubleVerify, suggested that buyers be on the lookout for abnormal spikes in traffic when deciding which sites to cut.

But as with most ad fraud tactics, the best way to avoid getting scammed is to avoid cheap inventory that sounds too good to be true.

“Buyers will think, ‘Woo-hoo! We reached a shitload of people,’” said ad fraud researcher Augustine Fou. “Yeah, except those were not human ‘people.’”