Hidden/stacked/covered or otherwise intentionally obfuscated ad serving
According to the Media Ratings Council, when multiple ads are delivered to the same page or app but are intentionally hidden – e.g, impossible for the user to see them – they are considered invalid ads.
Registering multiple clicks from the same user at the same time on multiple campaigns
Some fraudsters split this traffic over campaigns from different networks to evade detection. In these cases, time to install will have a flat distribution