diff --git a/includes/module.inc b/includes/module.inc
index 89f67c9..5a04382 100644
--- a/includes/module.inc
+++ b/includes/module.inc
@@ -730,8 +730,6 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
     else {
       $implementations = $implementations->data;
     }
-    // The cache may contain some implementations that are not defined, or that
-    // require an additional file to be included.
   }
 
   if (!isset($implementations[$hook])) {
@@ -755,16 +753,20 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
     if ($hook != 'module_implements_alter') {
       drupal_alter('module_implements', $implementations[$hook], $hook);
     }
-    // hook_module_implements_alter() may have changed the $group of some hooks,
-    // or added implementations that are not defined.
   }
 
+  // The first time a hook is used in a request, the implementations must be
+  // "verified" with function_exists(), and the file specified by $group must be
+  // included.
+  // This applies both to implementations loaded from cache, and implementations
+  // that were discovered just now and may have been changed with
+  // hook_module_implements_alter().
   if (empty($verified[$hook])) {
     foreach ($implementations[$hook] as $module => $group) {
       // If this hook implementation is stored in a lazy-loaded file, so include
       // that file first.
       if ($group) {
-        module_load_include($group . '.inc', $module);
+        module_load_include('inc', $module, "$module.$group");
       }
       // It is possible that a module removed a hook implementation without the
       // implementations cache being rebuilt yet, so we check whether the