Kimkia - 3:58 am on Aug 28, 2012 (gmt 0)
I got it too for two separate ad units on two different sites of mine (Incorrect site: googleads.g.doubleclick.net).
I call the first ad via SSI on a static html site; the second one is on a wordpress site. Both are fairly old ad placements that I haven't changed in a long time. I don't use iframes.
I'm just going to check now whether I have these units set up as an "alternate" on the one other major ad network that I use, where they would appear only if the other network has nothing available. I seem to think that is the case. Could this be where an iframe might be used? And could this be the issue for me?