Below are some useful tips and tricks to keep in mind when developing in WordPress.

Looping custom posts on the post screen

When looping posts on the admin screen, usually in the plugin, you'd be tempted to use normal 'The Loop':

if ($customQuery->have_posts()) {
  while ($customQuery->have_posts()) {
    $customQuery->the_post();
    // Stuff happens here
  }

  wp_reset_postdata();
}

There seems to be a bug in the core #18408. This bug affects the post data if you create a custom WP_Query object before the editor has been outputted. The post from this custom query will fill the edit post form. This happens because $wp_query->post is never defined in the admin load, which wp_reset_postdata() relies on to reset the original post data.

To remedy this you can loop through your posts by not setting the $customQuery->the_post().

if ($customQuery->have_posts()) {
  foreach ($customQuery->get_posts() as $post) {
    $postId = $post->ID;
  }
}

This will drop the need to reset post data, and the regular post will remain in the content.