diff -u b/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
--- b/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -307,6 +307,17 @@
     'type' => MENU_CALLBACK,
     'file' => 'taxonomy.pages.inc',
   );
+
+  // @todo Remove once drupal_valid_path() is fixed to find and access check
+  //   paths managed by the new routing system: http://drupal.org/node/1793520.
+  $items['taxonomy/autocomplete'] = array(
+    'title' => 'Autocomplete taxonomy',
+    // _menu_router_build() denies access to paths without a page callback.
+    'page callback' => 'NOT_USED',
+    'access arguments' => array('access content'),
+    'type' => MENU_CALLBACK,
+  );
+
   $items['admin/structure/taxonomy/%taxonomy_vocabulary_machine_name'] = array(
     'title callback' => 'entity_page_label',
     'title arguments' => array(3),