We are constantly updating and changing our websites. Web redesigns, deleting pages such as product pages and even spelling errors mean any links directing to the original URL will not be able to find that page as it no longer exists. This triggers a 404 Not Found error message to appear on the screen. These messages (errors) can be a nuisance to visitors, but don’t necessarily spell doom with the search engines.
In general, the search engines understand that 404 Not Found errors are going to happen. It becomes a problem when they happen more often than not. This sends a message to Google that you are not providing a good user experience and that your website is not being well managed. Ultimately, the search engines will lose confidence and your ranking will suffer.
404 Page Not Found messages are more of an issue from a user experience perspective. The goal obviously is to minimize these types of messages from occurring. But as stated above, 404 messages are going to happen. When they do, you have a chance to minimize the frustration by providing some useful information and an alternative redirect to your users. They are trying to find information, so first apologize for the page no longer being there. Then offer to send them to similar content elsewhere on your website.
Wondering what your 404 error page looks like? Type in www.YourURL).com/404 and you can see. Is it helpful? Does it make you want to continue on this site? If not, a change is in order.