At least in my country (Mexico), the advertisements in several ad networks like AdSense often contain malicious code which redirects you to a website telling:
"your version of WhatsApp is outdated, please update to the current version", and after a countdown (or the click of a link), it downloads a malicious apk to your phone.
I'm completely disappointed, since too many websites use AdSense, and sometimes I can't even check a website, because the redirection is too aggressive.
I understand that the people behind these shady advertisements use aggressive cloaking (ex. only injecting the malicious redirection when you are under a mobile network) to avoid being discovered.