A Verizon ad just did the same thing. These are the ads in the panel under the right sidebar menus in the HTML & CSS forum, seen on an individual thread. Click the screenshot thumbnail to enlarge it, and the ad stays where it is, on top of the screenshot.
But for some reason, the ad does not always load if you follow the link. But it does load if you follow the link, click the Web Development tab from top menu, and then hit the back button after the page loads.