For a long time Google Webmaster Tools has provided webmasters with a list of URLs that generate 404 pages. As Googlebot scours the web, it may encounter invalid links on your website. You can see what invalid links the Googlebot has encountered within the webmaster tool.
Since 2006, webmasters have been clamoring for Google to provide the source of the bad links as well. It is like a doctor telling you that you have a disease without providing any recommendations for treatment. I had recently voiced my opinion about this in the webmaster help group. I doubt if it was my recent complaint, but Google finally decided to add this feature.
Now when you view the list of invalid links, you will see a column labeled Linked From. In that column, you’ll see the number of pages that are linking to the invalid page. If you click on the hyperlinks in that column, you’ll get a popup that shows the sources of the invalid link. It is entirely possible that sources of bad links may be coming from other webpages. If that is the case, you may wish to contact the webmasters of those sites to change the URL to the proper page. An alternative to contacting webmasters would be to set up a redirect (301) to forward to the correct location. This is relatively easy to do if your webhost supports the Apache rewrite mod.