What is ad stacking and pixel stuffing?
Ad stacking and pixel stuffing are ways of serving ads nobody can see while still charging for them. Ad stacking layers several ads on top of each other in one slot so only the top one is visible but all count as impressions. Pixel stuffing squeezes an ad into a 1x1 pixel or tiny frame, so it technically loads but is invisible.
Both are forms of hidden ads and count as SIVT under the MRC IVT guidelines. They generate billable impressions on real devices with real users present, so they can slip past bot filters. Ad stacking is often found in apps and on low-quality sites; pixel stuffing can also hide video players playing ads in the background.
Viewability measurement exposes both: an ad that is stacked beneath others or rendered at 1x1 never reaches the MRC viewable standard. Buying on viewable impressions, and using ad verification that checks ad size and geometry, removes most of the incentive.