So I had this happen a few weeks ago, and a few times since, but got fed up today.
It's an Epson Brightlink ad. It covers the New Forum Posts box. I refreshed the page, and could see the box, but couldn't click on anything without taking me to the Epson page, even though the ad wasn't pulled down. Real annoying. If my mouse crosses that box at all, it drops down and covers stuff.
That ad the HUGE frontpage ad I see at the top that pushes everything on the page down almost out of browser windows. I believe it was Intel?