Landing page extension without 404 error
Like @Clive said, Drupal will try to resolve that path if it can. Just put a check in your page callback to see if there's an extra argument. If there is and it's valid the menu router will handle it. Otherwise it will fall back to your defined path, where if it finds something after /news, it will return drupal_not_found(). The 'return' is important, otherwise you might see the page render twice.

function MYMODULE_menu(){
    $items = array();
    $items['news'] = array(
       'title' =&gt; "Les news",
       'page callback' =&gt; 'landing_page',
       'access arguments' =&gt; array('access content'),
    );
    return $items;
}

function landing_page(){
    if (arg(1)) {
        return drupal_not_found();
    }

    return 'something';
}