I remember the day I figured out a Chrome plugin I regularly used was doing this. It was a Gmail notifier that had been widely used when I started using it. And after a while, I was getting the most annoying ads on YouTube without realizing that Google hadn't changed anything.

Rather, the plugin changed and added the ads to YouTube. It was so under-the-radar that I wouldn't have noticed unless I had decided one day to look up why YouTube was allowing allowing ads with sound to play over videos.

I'm a savvy user, but that was literally slipped under my nose. I think a lot of users that wouldn't have been nailed by such plugins in the past are starting to find them to be a huge problem.