Not Found Errors in Webmaster Tools as Ranked by
Priority, but what is
Priority based on?
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This was covered in a Webmaster Central blog post a few months ago.
One thing we’re really excited about in this new version of the Crawl errors feature is that you can really focus on fixing what’s most important first. We’ve ranked the errors so that those at the top of the priority list will be ones where there’s something you can do, whether that’s fixing broken links on your own site, fixing bugs in your server software, updating your Sitemaps to prune dead URLs, or adding a 301 redirect to get users to the “real” page. We determine this based on a multitude of factors, including whether or not you included the URL in a Sitemap, how many places it’s linked from (and if any of those are also on your site), and whether the URL has gotten any traffic recently from search.
It is, of course, based on "a multitude of factors" but the overall idea is that the errors are in rough order of what's most important and also likelihood you can actually do something about the problem. Scan down to the "Take action!" header there for some more detail.