diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index abda14e..44052ee 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -493,6 +493,9 @@ function hook_menu_get_item_alter(&$router_item, $path, $original_map) {
 /**
  * Define menu items and page callbacks.
  *
+ * Hey you! Hands off that page callback! Use the routing system! You can find
+ * out how to use it here: https://drupal.org/node/1800686.
+ *
  * This hook enables modules to register paths in order to define how URL
  * requests are handled. Paths may be registered for URL handling only, or they
  * can register a link to be placed in a menu (usually the Tools menu). A