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Google System reports that there’s a new tab in Google AdSense: Site Diagnostics. Google says this feature allows you to view “detailed diagnostic information about the pages Google crawls to display relevant ads.” In other words, you’ll learn if there were problems crawling a specific URL of yours to deliver targeted ads.
from: http://blog.outer-court.com/archive/2006-07-21-n84.html

What is the site diagnostic feature?

The Site Diagnostics page within your account provides details about whether our crawler was able to access your site in order to provide relevant ads. Our crawler sometimes encounters errors when attempting to crawl a page to determine its content. Using the site diagnostic section of your account, you can review the errors our crawler encountered for specific blocked URLs, then correct them in order for us to crawl and deliver the most relevant ads.