There seems to be a bug with the positioning of some of the ads when they're placed above the banner
Thanks for your report; I passed it on to our ad/ops team.
Looks like this might have been due to specific ads which didn't like to be restricted to the "frame" they should be restricted in. For a little while they managed to escape it, and busted out of it screaming FREEDOM! FREEDOM! before being reigned back into their framed prison where they belong.
Please do point out to us if something's awry. It'd be helpful if you could provide screenshots, browser, which ad etc... so for these kind of things webmaster@ is a lot better than a post on the forums.