The web works smoothly and users are satisfied as they browse through your website using your navigation menus and links. As long as they keep clicking on the navigation you've created you they shouldn't hit any error pages.
What is an error page anyway? 404 is the internal website code that is recorded when a page is not found. If a website were more personable it might say "the user wanted something but it wasn't on my website's hard drive, so I'm going to keep track of this request in a bucket I call 404."
Users of your website don't normally see 404/Error Pages as long as they click on the correct links and do what you expect them to do. Since the web is so social people will share links to their friends through email, social networks, and even text messages. You have other people who bookmark your pages on their browser or through their favorite bookmarking application.
Over time you may decide that your website needs a tune-up, or you want to change things around. Every time you delete a website page, a product, or change the date on one of your blog posts you run the risk of breaking one of those bookmarked or shared links that are somewhere out in cyberspace. This is where the term "Broken Links" come from, because you have done something on your website which killed the links to you. SEO guys, me included, usually get upset when you do this without planning ahead to prevent the breaks. You fix broken links with a 301 and a 302, but I'm not talking about that today.
Bookmarked pages in a personal web browser are not that big of a deal because personal bookmarks only affect one person. On the other hand, when users click public bookmarks through Delicious, Digg, Reddit, and StumbleUpon they will suddenly see the 404/Error Page on your site instead of the original page.
Realize that all the shared links from Twitter, Facebook, Google+, and LinkedIn will also break when you change your website. Those links are far more important than the bookmarking services because more people are inclined to revisit these links, especially if you've had good viral content.
In order to prevent broken links you will need to track your website changes or have a sophisticated website that does it for you. Regardless if you can track broken links or not you never want to show your website users the infamous "404 Error Page Not Found" message. Instead you want to show them a friendly message that says you "moved or changed the website."
I see a big opportunity for marketing on the 404 pages. By default most websites are initially set up to actually say "404 Error Page Not Found," but you should change that to a customized page that you can control from within your content management system.
Your website Error Page is something you need to set up and control, but it's not something you need to change if you pre-program it with these ideas:
1. Set up your Error Page design and layout to look exactly like the rest of your website.
2. Have your current sale items or monthly specials appear automatically.
3. Include basic details of your business and links to popular pages within your website. Even though this page will have your normal navigation you should including these links in the body of the page by saying "You might want to check out these popular pages: ______"
4. Include a link to your blog.
5. Include your Twitter stream.
6. Include a feed from your Facebook
With these 6 ideas you can turn your Error Page from a customer disappointment to a potential for further engagement.