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

Question

I\'m adding validation so if a post is in a particular category, it needs certain Custom Fields to be set.

This should be easy hooking wp_insert_post_data

Answer 1

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

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