Ad stacking

In ad stacking, a page loads many ads into the same position like a pile of posters. The visitor sees only the top one. Every ad in the pile reports that it was shown, and sometimes a single click is registered against several of them. Google lists ad stacking among its named types of invalid activity.

The technique belongs mainly to display and video advertising, where payment is per impression. It matters to search arbitrageurs for two reasons. First, some low-quality sites that sell cheap traffic fund themselves this way, which is a warning about the quality of the audience they send. Second, a lander that stacks or overlays feed units, or renders ad frames behind other content, breaks feed rules about modifying and placing the code.

Signs include far more ad requests than a page could physically display, impressions reported for ads with zero visible area, and one user action producing several clicks.

The defence for a buyer is to inspect where traffic really comes from and to prefer sources with independent measurement. Related techniques are hidden ads and domain spoofing.

Think of it like this

It is a billboard owner pasting ten posters on top of each other and billing all ten advertisers for the same wall.