Improperly Handled 404 Error Pages Negatively Impact SERPs

[Kristijntje]

Without a custom 404 error page, the visitor -- human or robot -- is left with only two courses of action: to abandon their search or click the back button. Search engines can reduce rankings due to server errors and broken pages. Simple errors such as "404 page not found" in large quantities can make the search engines believe that a site isn't complete or is under construction and, as a result, they may determine that the site isn't worthy of strong search engine rankings.

When a nonexistent page is requested from the server, the server should respond with a special "HTTP Status" header value of "404 Not Found," which may also be followed by custom error-page body content. Incorrectly configured Web-servers that respond with a status header value of "200" (or any other erroneous value) are exposed to significant risk with respect to search engines' "duplicate content penalties." This is because the identical content (in this case, the error page content) would be available under a potentially infinite number of URLs.

Custom 404 pages serve several important purposes. First, they return the correct code to the users and to search engine spiders, informing the visitors that the page they were seeking wasn't found. Second, custom 404 pages present visitors with options about what to do next...

full article: http://searchenginewatch.com/showPage.html?page=3631002