Say in the YouTube app you perform a search for a song. The results come back very quickly and the first result is what you want so you tap it. An ad starts to play. Except its not an ad. Its a video from the advertised spot above your search results - wait a minute you didn't click that. So you go back and realize, it loads 1 second after your search results and pushes them downwards, so your tap ends up on the advertisement that wasn't even rendered yet. Hm.