reverted:
--- b/core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php
+++ a/core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php
@@ -54,8 +54,9 @@
     }
     $definition = $typed_config->getDefinition($config_name);
     $this->schema = $typed_config->create($definition, $config_data);
+    $errors = array();
     foreach ($config_data as $key => $value) {
+      $errors = array_merge($errors, $this->checkValue($key, $value));
-      $errors = $this->checkValue($key, $value);
     }
     if (empty($errors)) {
       return TRUE;