When Googlebot crawls your site, it’s expected that most URLs will return a 200 response code, some a 404 response, some will be disallowed by robots.txt, etc. Whenever Google is unable to reach your content, it shows this information in the Crawl errors section of Webmaster Tools (even though it might be intentional and not actually an error). Continuing with this effort to provide useful and actionable information to webmasters, Google now sends SiteNotice messages when they detect a significant increase in the number of crawl errors impacting a specific site. These notifications are meant to alert you of potential crawl-related issues and provide a sample set of URLs for diagnosing and fixing them.
A SiteNotice for a spike in the number of unreachable URLs, for example, will look like this:
This SiteNotices will be helpful to you for discovering and dealing with issues that, if left unattended, could negatively affect your crawl coverage. You’ll only receive these notifications if you’ve verified your site in Webmaster Tools and they detect significant changes to the number of crawl errors it encountered on your site. And if you don’t want to miss out on any of these important messages, you can use the email forwarding feature to receive these alerts in your inbox.