There are a few things you should keep in mind when building your 404 pages:
No advertising of any kind.
All error pages should be free of service-calls, such as advertising modules. This means 404 pages should be static HTML and not include any complex script, advertising or anything which makes calls off the page itself. This is due to the risk of the requested resource not returning in a timely manner which could lead to loss of platform integrity (causing a server to crash).
The low volumes these 404 pages experience, combined with the goal of signposting users back to content and the goal of acting as an “error” page, mean that serving advertising is not beneficial to the user experience, not going to generate significant impressions and will ultimately drive down overall ad yield and value due to the very low click-through rates this inventory generates.
This also includes the insertion of automated widgets that can return search results related to the content of the original page. Such widgets can end up crawled and creating a loop is items served in the search results, themselves, return 404 error pages. This can hurt a search crawler, causing it to avoid you site in the future, and can also harm your own servers.
Page returns a 404 status code
From an SEO perspective, a 404 page should return a 404 Status Code (Page Not Found) as opposed to a 200 (OK) status code. The return of a 404 Status Code is used to alert automated users such as search engine crawlers that the link is in fact broken, and is the only way an automated user can ascertain this. If 404 pages return a 200 Status Code then Search Engines consider this broken link still valid, and the “404 page” could end up in the index.
As mentioned above, the goal of the 404 pages is two-fold. First to alert the visitor that the content is no longer there, and second to offer a way for that visitor to re-engage with your website.
It is acceptable to have a 404 page which is matched to the visual layout of your website, and that displays other options which a visitor might click on to find related content across your website. Many websites showcase a series of links to their most popular content on the 404 page to keep the visitor from leaving the website.
In all cases, be sure to make a 404 page which is light-weight and loads as quickly as possible. Even “smart” 404 pages should avoid external calls to services which populate modules inside the page with data. House the information directly within the page itself.
Source : www.binglinks.org