About Not Found errors
Google discovers content by following links from one page to another. Generally, a Not Found status error (usually a 404 HTTP status code) is returned when Googlebot attempts to visit a page that doesn’t exist—either because you deleted or renamed it without redirecting the old URL to a new page, or because of a typo in a link.
Dealing with Not Found errors
To see where invalid links are coming from, click the URL. In the error dialog, click the Linked from these pages tab.
If the links are coming from your own site, fix or delete them. If they’re coming from an external site, you can use this data to help improve your site’s user experience. For example, a misspelling of a legitimate URL (www.example.com/awsome instead of www.example.com/awesome) probably happens when someone intended to link to you and simply made a typo. Instead of returning a 404, you could 301 redirect the misspelled URL to the correct URL and capture the intended traffic from that link. You can also make sure that, when users do land on a 404 page on your site, you help them find what they were looking for rather than just saying “404 Not found." However, this is only worth the effort if the incorrect link is generating significant traffic.
You can also contact the webmaster of a site with an incorrect link, and ask for the link to be updated or removed.
404s are a perfectly normal (and in many ways desirable) part of the web. You will likely never be able to control every link to your site, or resolve every 404 error listed in Webmaster Tools. Instead, check the top-ranking issues, fix those if possible, and then move on.
When to return a 404 status code
When you remove a page from your site, think about whether that content is moving somewhere else, or whether you no longer plan to have that type of content on your site.
If you’re moving that content to a new URL, you should 301 redirect the old URL to the new URL—that way when users come to the old URL looking for that content, they’ll be automatically redirected to something relevant to what they were looking for.
If you’re getting rid of that content entirely and don’t have anything on your site that would fill the same user need, then the old URL should return a 404 or 410. Currently Google treats 410s (Gone) the same as 404s (Not found).
Returning a code other than 404 or 410 for a non-existent page (or redirecting users to another page, such as the homepage, instead of returning a 404) can be problematic. Such pages are called soft 404s, and can be confusing to both users and search engines.
You can improve the user experience by configuring your site to display a custom 404 page when returning a 404 response code. For example, you could create a page containing a list of your most popular pages, or a link to your home page, or a feedback link. You can also use the Webmaster Tools Custom 404 widget to add a search box and more site search options to your site. But it’s important to remember that it’s not enough to just create a page that displays a 404 message. You also need to return the correct 404 or 410 HTTP response code.