strange behaviour of template_redirect in IE8

I have implemented page not found with a custom template and used template_redirect for it.

add_action("template_redirect", array($this, "redirection_section"), 11);

Now everything is working fine as expected but not in IE8, my redirect section method is here:

function redirection_section() {
        global $is_IE;

        if($is_IE) {
            echo "GOT IT!";
            exit;
        }
        if( is_404() ) {
            $this->uni_page_redirect();
        }
}

if i type like anything in the end of the url like myurl.com/testing its displaying me

The webpage cannot be found

But if i call other menus or template or and templated related stuff then got the proper GOT IT message, But why not with myurl.com/testing? Can anyone tell me what is the reason or do i have to implement something else? Please guide me to the right direction.