diff --git a/includes/module.inc b/includes/module.inc
index 5a04382..0c06609 100644
--- a/includes/module.inc
+++ b/includes/module.inc
@@ -676,6 +676,10 @@ function module_hook($module, $hook) {
 /**
  * Determines which modules are implementing a hook.
  *
+ * Lazy-loaded include files specified with "group" via hook_hook_info() or
+ * hook_module_implements_alter() will be automatically included as part of
+ * module_implements(*, *, FALSE).
+ *
  * @param string $hook
  *   The name of the hook (e.g. "help" or "menu").
  * @param bool $sort
@@ -696,7 +700,7 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
   static $drupal_static_fast;
   if (!isset($drupal_static_fast)) {
     $drupal_static_fast['implementations'] = &drupal_static(__FUNCTION__);
-    $drupal_static_fast['verified'] = &drupal_static(__FUNCTION__ . '.verified');
+    $drupal_static_fast['verified'] = &drupal_static(__FUNCTION__ . ':verified');
   }
   $implementations = &$drupal_static_fast['implementations'];
   $verified = &$drupal_static_fast['verified'];
@@ -722,6 +726,9 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
   }
 
   // Fetch implementations from cache.
+  // This happens on the first call to module_implements(*, *, FALSE) during a
+  // request, but also when $implementations have been reset, e.g. after
+  // module_enable().
   if (empty($implementations)) {
     $implementations = cache_get('module_implements', 'cache_bootstrap');
     if ($implementations === FALSE) {
@@ -730,6 +737,10 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
     else {
       $implementations = $implementations->data;
     }
+    // Forget all previously "verified" hooks, in case that $implementations
+    // were cleared via drupal_static_reset('module_implements') instead of
+    // module_implements(*, *, TRUE).
+    $verified = array();
   }
 
   if (!isset($implementations[$hook])) {
@@ -748,20 +759,31 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
         $implementations[$hook][$module] = $include_file ? $hook_info[$hook]['group'] : FALSE;
       }
     }
-    // Allow modules to change the weight of specific implementations but avoid
+    // Allow modules to change the weight of specific implementations, but avoid
     // an infinite loop.
     if ($hook != 'module_implements_alter') {
+      // Remember the implementations before hook_module_implements_alter().
+      $implementations_before = $implementations[$hook];
       drupal_alter('module_implements', $implementations[$hook], $hook);
+      // Verify implementations that were added or modified.
+      foreach (array_diff_assoc($implementations[$hook], $implementations_before) as $module => $group) {
+        // If drupal_alter('module_implements') changed or added a $group, the
+        // respective file needs to be included.
+        if ($group) {
+          module_load_include('inc', $module, "$module.$group");
+        }
+        // If a new implementation was added, verify that the function exists.
+        if (!function_exists($module . '_' . $hook)) {
+          unset($implementations[$hook][$module]);
+        }
+      }
     }
+    // Implementations for this hook are now "verified"
+    $verified[$hook] = TRUE;
   }
-
-  // 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[$hook])) {
+  elseif (!isset($verified[$hook])) {
+    // Implementations for this hook were in the cache, but they are not
+    // "verified" yet.
     foreach ($implementations[$hook] as $module => $group) {
       // If this hook implementation is stored in a lazy-loaded file, so include
       // that file first.