diff -u b/core/lib/Drupal/Core/Config/ConfigFactory.php b/core/lib/Drupal/Core/Config/ConfigFactory.php
--- b/core/lib/Drupal/Core/Config/ConfigFactory.php
+++ b/core/lib/Drupal/Core/Config/ConfigFactory.php
@@ -243,6 +243,9 @@
   public function getCacheKeys() {
     $keys = array();
     if ($this->useOverrides) {
+      // Because get() adds overrides both from $GLOBALS and from
+      // $this->configFactoryOverrides, add cache keys for each.
+      $keys[] = 'global_overrides';
       foreach($this->configFactoryOverrides as $override) {
         $keys[] =  $override->getCacheSuffix();
       }