Google has now implemented a new feature “Crawl error sources” in Webmaster Tools, that makes the process of tracking down the causes of "Not found" errors easier. You can now use crawl error sources to quickly determine if the cause is from your site or someone else's. You'll have the information you need to contact them to get it fixed, and if needed, you can still put in place redirects on your own site to the appropriate URL.
- In the "Not Found" and "Errors for URLs in Sitemaps" reports, Google've added the "Linked From" column. For every error in these reports, the "Linked From" column now lists the number of pages that link to a specific "Not found" URL.
- Clicking on an item in the "Linked From" column opens a separate dialog box which lists each page that linked to this URL along with the date it was discovered. The source URL for the 404 can be within or external to your site.
You can also download all your crawl error sources at once. Just click the "Download all sources of errors on this site" link to download all your site's crawl error sources.