In June, Google began showing a notice of faulty redirects within the Google search results page that told the searchers that if they click on that search results from their smartphone, it may take them to the site's home page instead of the page they are expecting.
A couple months later, Google has begun notifying the webmasters that this impacts. So in June, the searchers were shown a prompt in the search results. But now, webmasters are being notified of this via Google Webmaster Tools.
A Google Webmaster Help thread has one webmaster who received the notice, it reads:
Stop redirecting smartphone users to the homepage of domain.com
Dear webmaster of http://www.domain.com/,
Google systems have detected that your desktop site URLs redirect smartphone users to the homepage of the smartphone site rather than the page they found in Search results. This critical issue causes problems for Googlebot for smartphones because it can’t access your smartphone URLs. It also confuses your users.
@Marie_Haynes has a nice screen shot of this as well:
I am a bit upset they didn't send out these warnings prior to implementing the fault redirect prompt in the search results. This would have given webmasters time to implement it correctly and not lose click throughs from the search results.
But I guess, better late than never?
Forum discussion at Google Webmaster Help.