Recently, I started actually going through Google Webmaster tools and looking at the "Not Found" errors. A couple were legitimate, but I found a ton that made no sense. Here is an example:
There are over 100 "Not Found" errors for links that should not exist. When I go to the page and look at the source, this link is nowwhere to be found.
Stranger yet, the link is always show to be found on the correct page URL for the bad URL. So Webmaster Tools would say:
is linked to from