Tristan Savatier, aka loupiote, spotted an interesting issue with Google indexing and displaying Bing's image cached results in the Google Image search index.
He posted a thread at Google Webmaster Help explaining that he found 8 million or so of the Bing search-engine cached thumbnail images being indexed by Google Image search. He then linked to this allinurl command that shows images that contain "mm.bing.net/images/thumbnail.aspx" in the URL.
New Googler, Gary Illyes, explained the reason Google is indexing the content is because neither Bing or the original site are blocking Google from indexing it.
Two issues with this:
(1) Google has gone on record about them not wanting to have search results in their search results.
(2) Remember a few months ago, Google accused Bing of cheating because they were sniffing their keyword searches and results?
As the forum complaint went:
I am not so sure that it's a good idea to index the cache thumbnails of another image search engine.
Do you think it would be a good idea if bing was indexing all the thumbnail images from the google-image cache image database?
Forum discussion at Google Webmaster Help.
Update: Google seems to have now removed the images despite Bing not preventing the crawl of those images in the robots.txt file.
A Google spokesperson sent us the following statement:
Like other search engines, we regularly crawl the web and index pages, images and other content unless it's blocked by the industry standard robots.txt file. A user discovered that Bing had failed to block some image thumbnails using robots.txt, so these thumbnails were automatically indexed during our usual crawl process. In our webmaster guidelines we ask site owners to use robots.txt to prevent crawling of auto-generated pages that don't add much value for users coming from search engines, so we've taken action to remove these thumbnails from our index, consistent with that policy.