Dread it and run from it, at one point or another Error 404 pages will still pop up on your site, and for various reasons. This is hard to avoid, particularly for websites where content is constantly changing or being updated. It is also normal to see these pages when a website undergoes a major overhaul and the layout of the URLs is altered.
The “404 Error” page is a response code from a server when a web browser tries to open a page that the server couldn't locate. Notwithstanding this, not all 404 pages harm your website. This code can actually be useful when it comes to troubleshooting problems.
infographic by: blog.spiralytics.com