Define your own 404 page.

If a page is not found (either it's been renamed or removed, or maybe the user entered the wrong URL), the server generates a 404 message, which most browsers interpret and display a generic page with a "page not found" message.

By trapping that error condition and showing your own "page not found" message, you have the opportunity of still showing your own contact information and navigation options. A much less confronting situation for the visitor that can offer the option of them easily finding their way to the correct page rather than abandoning your website.