I get this randomly while visiting threads here.
It only happens here on this site, and I've done a complete sweep of my system. As you can see it replaces the whole page. And whats worse is if you let it go, it keeps trying to trick you into installing something.
I dunno if it is tied to an ad, or if there is something on my system that got missed. But like I said it only happened 3 times so far, and each time I was thread jumping here.