I think it may have been for some photo bundle(?). It appears to be gone now, but it literally covered almost the entire screen. It "popped up" and stayed for a bit, then disappeared, then if I scrolled back up, even just a little bit, it would reappear.
There is a re-occurring one for Home Depot that pops up on AVS Forums that is beyond obnoxious, because it covers a large portion of the screen with no way to remove it or get around it, except for returning to the previous topic page. But this one put the Home Depot/AVS Forums pop-up to shame.
I understand running a forum takes money, like running anything else. But if the ads hinder the usefulness of the forums to the users and inhibit them from having a good experience, the users will be driven away. And no user traffic means no ad revenue. So a happy medium and realistic balance must be struck.