A WebmasterWorld thread has a report from an SEO of a new Google Webmaster Tools alert.
The new alert is named "Detected New Page" and warns the webmaster that a specific URL was replaced by another URL when Google feels it has been replaced.
Why would Google think it would have been replaced? Either a 301 or 302 redirect was set up. Maybe the content is too similar to another page?
The message reads something like this:
The URL www.pandalized.com/page.html is no longer appearing in Google search results because our algorithms have selected www.freshsite.com/page.html instead. Please see this Help Center article for more information about this message.
This is the first report I have seen of such an error in Google Webmaster Tools message center.
Forum discussion at WebmasterWorld.