Wordpress: displaying an error message - hook admin_notices fails on wp_insert_post_data or publish_post

You can simply do like WordPress do : using transients like this :

function set_post_pending($data, $postarr) {    // If it's not valid...        $error = "You are missing some Custom Fields.";        set_transient( get_current_user_id().'missingfield', $error );        $data['post_status'] = 'pending';    return $data;}add_filter('wp_insert_post_data', 'set_post_pending',1,2);function show_admin_notice() {    if($out = get_transient( get_current_user_id().'missingfield' ) ) {        delete_transient( get_current_user_id().'missingfield' );        echo "<div class=\"error\"><p>$out</p></div>";    }    // return false; // nothing to return here}add_action('admin_notices', "session_admin_notice");

ps : avoid $_SESSION in WordPress, thx

Wordpress doesn't use sessions, and if register_globals is on it will clear the $_SESSION array.

Wordpress passes it's messages along using a message integer in the URL, an array of messages is then defined in the relevant edit-[type]-form.php file in the wp-admin folder. I think you could probably append your own variable to the redirect and then get that in your admin_notices hook function. Take a look at the edit-[type]-form.php files to get an idea of how this might work.

if($out = $_SESSION['admin_notices']) {       $_SESSION["admin_notices"] = "";       echo $out; }

This condition is always TRUE so it always reset your $_SESSION['admin_notices'] var