Custom fields and auto save

You need to check to see whether the $_POST data contains your custom field information before saving it:

  if (!empty($_POST["intro"])) {
    update_post_meta($post->ID, "intro", $_POST["intro"]);
  }

Also, WordPress will pass the post ID and post object into your callback if you ask it too.

function save_intro($ID, $post){ //preserve the data in the admin section
 // ...
}
add_action('save_post', 'save_intro',10,2);

You don’t need that global $post at all.