Contributed Field Widget in Custom Form

rdg-blog-394-a.php

<?php
$field = field_info_field('field_some_youtube_video');
$instance = field_info_instance('node','field_some_youtube_video','custom_video');
$items = array();
if (isset($node->field_some_youtube_video[LANGUAGE_NONE][0])) {
  $items = $node->field_some_youtube_video[LANGUAGE_NONE];	
}
$form['#parents'] = array(); // a required element to use 'field_default_form' function below.
$video_field = field_default_form('node', $node, $field, $instance, LANGUAGE_NONE, $items, $form, $form_state);
$form['video'] = $video_field['field_some_youtube_video'];