A 404 error message is a response delivered via an HTML document indicating that the request was unable to communicate with the server. Usually this is caused by a request not being able to locate a file or a link is broken on a site. Lets say you put up a site and when a spider crawls it, for some reason, the page is not loaded, requested or the link is broken. The unoptimized 404 error page will KILL the potential site spidering causing a site to not rank in the search engines. This can also cause a novice user to get frustrated and abandon your site.
One of the best 404 error pages I have seen created is Searchenginewatch.coms 404 error page. The searchenginewatch.com error page implements their entire navigation, search box, RSS feed, advertisers, links to popular posts, news links and more. This means if a spider finds a 404 error page the spider will continue crawling and indexing the entire site without getting stopped by an unoptimized 404 page.
Some of the ways 404 pages are reached are:
Bookmarked sites that have since been moved
Made an error when typing in a url
An moved page is still indexed in the SERPS
Broken links in your link structure
What are some tips when customizing your 404 error pages?
1. Put a link to your FAQ page
2. Put a link to your top level categories
3. Put a link to your sitemap
4. Create a templated 404 page that blends with your site
5. Add a search box
6. Make your 404 pages look as close to your site theme as possible
7. Add a true navigation to your site.
Optimizing your 404 error page is one of the most overlooked SEO techniques I have seen in the industry. When doing SEO it is easy to become overwhelmed with doing your on-page and off-page optimization but failure to optimize this can lead to problems down the road and more importantly a tarnishing reputation.