diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 9353a76..1340e5b 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -1198,6 +1198,12 @@ public function executeHandlers($type, &$form, &$form_state) {
    * {@inheritdoc}
    */
   public function setErrorByName($name, array &$form_state, $message = '') {
+    // Errors must be set during validation only.
+    // @todo Move $this->validatedForms into $form_state.
+    if (isset($form_state['build_info']['form_id']) && !empty($this->validatedForms[$form_state['build_info']['form_id']])) {
+      throw new \BadMethodCallException('Validation errors cannot be set during submission handling.');
+    }
+
     if (!isset($form_state['errors'][$name])) {
       $record = TRUE;
       if (isset($form_state['limit_validation_errors'])) {