Earlier today, we reported that over 4 million Google AdSense ads were being indexed by Microsoft Live Search. Tonight, the Microsoft Live Search Webmaster Center blog has posted that they’ve identified and fixed the issue and that search results should return to normal within the next few days.

What happened? Google AdWords URLs are blocked by Google’s robots.txt file with this command:

Disallow: /pagead/

The blog post explains that when Live Search finds links to pages that are blocked by robots.txt, they may index the link (as listed URL) and anchor text (as the listed title). Google and Yahoo do the same. Google calls these results “partially indexed URLs” and Yahoo! calls them “thin listings.” Live Search found these AdWords URLs as links in AdSense blocks on sites they crawled.

In this case, AdWords URLs were listed not only because of the way Live Search indexes blocked URLs, but also because of a change in how Google renders AdSense units. However, the blog post doesn’t describe this change and the Google AdSense blog doesn’t mention changes.

Microsoft recently posted about another issue with their crawler that caused them to download AdSense blocks, which may have skewed ads reporting. Likely this latest issue wouldn’t impact ad reporting, as the bot crawling the AdSense links identified itself as MSNbot.

Related Topics: Google: AdSense | Google: AdWords | Microsoft: Bing SEO

About The Author: Vanessa Fox is a Contributing Editor at Search Engine Land. She shares her perspectives on how search impacts marketing, user experience, and web development at ninebyblue.com. Her book, Marketing in the Age of Google, provides a blueprint for incorporating search strategy into organizations of all levels. Follow her on Twitter at @vanessafox. See more articles by Vanessa Fox