There are cases where user type an invalid page URL within you'r website. You can provide the user with an custom error page saying that the page does not exists in the website.
Use the web.config to point to the custom page.
<customErrors mode="On" defaultRedirect="GenericErrorPage.htm">
<error statusCode="404" redirect="FileNotFound.htm"/>
</customErrors>
Error status code 404 - Page not found