- There is a mistyped URL, or a copy-and-paste mistake
- Broken or truncated links on web pages or in an email message
- Moved or deleted content
Usually when the visitors come across such a page not found message they usually click the back button or navigate away from the site and the website may as a result lose on some targeted traffic.
A 404 page is a standard HTML page and can be customized according to your need. You can customize it the way you want but there are certain points to be kept in mind so that it can be made useful and it can serve the purpose of making the visitor go to the relevant existing page.
When we say you can create it the way you want the emphasis is not on the design but on the message and link to the new site or relevant page so that your site does not lose out on targeted visitors.
Ideally the design should be the same as the rest of your site and more important with all the links and navigation to the other pages so the visitor can navigate to the rest of the site from that page too.
In order to enhance the functionality you can add a search box to that page and of course the link to the home page or the relevant page has to be there.
Google strongly recommends to have a customized 404 page and also offers guidelines on how to create it http://www.google.com/support/webmasters/bin/answer.py?answer=100044&hl=enas these are small things that matter much for your website.
A few examples: