diff --git a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
index 904e59c..1c4c23a 100644
--- a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
+++ b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
@@ -12,6 +12,8 @@
 use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 use Drupal\Core\Form\FormAjaxException;
 use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormBuilder;
 use Drupal\Core\Form\FormBuilderInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent;
@@ -68,9 +70,11 @@ public function onView(GetResponseForControllerResultEvent $event) {
    */
   public function onException(GetResponseForExceptionEvent $event) {
     $exception = $event->getException();
+    $request = $event->getRequest();
 
-    // @todo Comment.
-    if ($exception instanceof BadRequestHttpException) {
+    // Render a nice error message in case you have a file upload which exceeds
+    // the configured upload limit.
+    if ($exception instanceof BadRequestHttpException && $request->query->get(FormBuilder::AJAX_FORM_REQUEST)) {
       drupal_set_message($exception->getMessage(), 'error');
       $response = new AjaxResponse();
       $status_messages = array('#type' => 'status_messages');
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index f31471e..fd3c4a7 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -265,7 +265,9 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
     // can use it to know or update information about the state of the form.
     $response = $this->processForm($form_id, $form, $form_state);
 
-    // @todo Comment.
+    // In case the post request exceeds the configured allowed size
+    // (post_max_size), the post request is potentially broken. Add some
+    // protection against that and at the same time have a nice error message.
     if ($ajax_form_request && !isset($form_state->getUserInput()['form_id'])) {
       throw new BadRequestHttpException(t('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@size) that this server supports.', array('@size' => format_size(file_upload_max_size()))));
     }
diff --git a/core/modules/file/src/Tests/FileManagedFileElementTest.php b/core/modules/file/src/Tests/FileManagedFileElementTest.php
index 8368610..1bd46c0 100644
--- a/core/modules/file/src/Tests/FileManagedFileElementTest.php
+++ b/core/modules/file/src/Tests/FileManagedFileElementTest.php
@@ -6,6 +6,8 @@
  */
 
 namespace Drupal\file\Tests;
+use Drupal\Component\Utility\SafeMarkup;
+use Drupal\file\Entity\File;
 
 /**
  * Tests the 'managed_file' element type.
@@ -106,6 +108,31 @@ function testManagedFile() {
 
             $this->drupalPostForm(NULL, array(), t('Save'));
             $this->assertRaw(t('The file ids are %fids.', array('%fids' => '')), 'Submission after file upload and removal was successful.');
+
+            // Upload a file which exceeds the configured post_max_size limit.
+            $post_max_size_init = ini_get('post_max_size');
+            if ($post_max_size_init) {
+              // Parse the upload size in order to produce a file bigger as it.
+              $regex = '/^([0-9]+)([bkmgtpezy])$/i';
+              preg_match($regex, $post_max_size_init, $match);
+              $post_max_size = round($match[1] * pow(1024, stripos('bkmgtpezy', strtolower($match[2]))));
+              $big_test_data = str_repeat('a', $post_max_size * 1.1);
+              file_put_contents('public://test.dat', $big_test_data);
+
+              /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+              $file_system = \Drupal::service('file_system');
+              $edit = array($file_field_name => $file_system->realpath('public://test.dat'));
+              if ($ajax) {
+                $this->drupalPostAjaxForm(NULL, $edit, $input_base_name . '_upload_button');
+                $this->assertText(SafeMarkup::format('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@post_max_size) that this server supports.', ['@post_max_size' => $post_max_size_init]));
+              }
+              else {
+                // @todo Currently the post request just fails, as there is no
+                //   input, so I guess we can't protect against it? This would
+                //   be odd.
+                $this->drupalPostForm(NULL, $edit, t('Upload'));
+              }
+            }
           }
         }
       }