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Essentially, our search algorithm saw a large area on the blog that was due to an IFRAME included from another site and that looked spammy to our automatic classifier. I believe that this bug has been fixed now. We also added additional safety checks to the relevant system that would escalate to an engineer if this site had the same issue in the future.
Another thread here, also relevant to the topic: Does iFrame impact Google SERP [webmasterworld.com]
The facebook widget that several people use on their blogs is based on iframe...but no one complained so far...
maybe it was slow loading, dragging the speed of the page down.
I don't know if this is relevant, or even the correct protocol, but I no follow the iFrame code as the content would be meaningless in SERPS as standalone data.
<div rel="nofollow" etc etc>Content with links</div>