netmeg - 8:46 pm on Dec 27, 2012 (gmt 0)
I see them occasionally, in different circumstances.
I always figured it was because there wasn't enough ad inventory for the page, because the problem seemed to go away when I went from 3 ad units to two.
But lately I've been seeing them a LOT with remarketing (where the ads from a site you've visited follow you around the net) where they put the remarketing ads on every AdSense spot on the page.
Again, bad advertisers and a bad delivery system.