Every day you visit one of them. The pages, which don't exist any longer, have moved to a new server or have never existed at all. Once you've missed your intended destination, either through a bad or outdated link, or a typo in the page you were hoping to reach, you'll hopefully get an internal error message from the server. However, by default these messages aren't that helpful - after all, you are looking for the information, not for the reason you can't find this information. Good news ...