I just recently had my phone hijacked by absolutely ridiculous adware, thanks to a previously trusted app going selling out to a vicious ad system. I've always liked having a calculator widget and was a fan of a particular one. Free, looked good, and functioned well. Used it for quite a while across multiple devices. Suddenly, following an automatic update, I turned on the phone one afternoon to find a disclaimer on my screen as long as my arm. Tried to just cancel and move on, but that didnt work. Within a few hours, my homescreens started filling up with links to app downloads of all sorts. Delete those and new ones would reappear almost instantly. Few hours later, started getting text message notifications that showed up constantly (several per hour) for everything from get-rich quick scams to porn. I was pissed.
Finally I put two and two together and realized what had happened. All of this mess was being brought by an "ad" company called "AirPush". Thank goodness some kind soul has an app on Google Play called "AirPush Detector" that leads you to the rogue app, and helps eradicate AirPush from the phone w/o having to do a reset. Sure enough...the former trusted calculator app was the guilty app. They were once one of the highest rated calculator apps on the Google. Since this debacle, they are slammed with negative reviews over the adware.