The proper server responses dictate how search engines will treat missing or redirected pages. When checking to see if your site handles a “404 Page Not Found” response, it’s important to check pages outside the URL structure AND pages within the URL structure.
Make sure they don’t 302 redirect to the 404 page, and then return a 200 HTTP status code. Some websites report a “not found” error by returning a standard web page with a “200 OK” response code; this is known as a soft 404.
Both URL patterns don’t return the proper 404 header response when accessing a page outside or inside the URL structure of the site. Both should return a 404 page header response.
In addition to ensuring that the proper header response is being returned, it’s important to think about the user-experience. Once a visitor lands on your 404 page, are they lost? Do they just leave your site? Or, is there some kind of alternative navigation they can use to get back on track, and find what they were looking for, and stay on your site.
Customize the 404 error to be like that of the proposed HTML sitemap. Deliver a custom 404 page that contains links to additional parts of the site. This allows search spiders and users to continue moving on the site even when they reach a non-existing page. Keeping in mind a good user-experience, and in some cases you might want to add some humor to your 404 page.
Here is a list of some of the best 404 pages on the web.