diff --git a/core/lib/Drupal/Core/Form/FormValidator.php b/core/lib/Drupal/Core/Form/FormValidator.php
index d7d70f3503..2516f174e0 100644
--- a/core/lib/Drupal/Core/Form/FormValidator.php
+++ b/core/lib/Drupal/Core/Form/FormValidator.php
@@ -335,6 +335,8 @@ protected function performRequiredValidation(&$elements, FormStateInterface &$fo
     }
 
     if (isset($elements['#options']) && isset($elements['#value'])) {
+      $name = empty($elements['#title']) ? $elements['#parents'][0] : $elements['#title'];
+      $message_arguments = ['%name' => $name];
       if ($elements['#type'] == 'select') {
         $options = OptGroup::flattenOptions($elements['#options']);
       }
@@ -345,8 +347,9 @@ protected function performRequiredValidation(&$elements, FormStateInterface &$fo
         $value = in_array($elements['#type'], ['checkboxes', 'tableselect']) ? array_keys($elements['#value']) : $elements['#value'];
         foreach ($value as $v) {
           if (!isset($options[$v])) {
-            $form_state->setError($elements, $this->t('An illegal choice has been detected. Please contact the site administrator.'));
-            $this->logger->error('Illegal choice %choice in %name element.', ['%choice' => $v, '%name' => empty($elements['#title']) ? $elements['#parents'][0] : $elements['#title']]);
+            $message_arguments = $message_arguments + ['%choice' => $v];
+            $form_state->setError($elements, $this->t('The submitted value %choice in the %name element is not allowed.', $message_arguments));
+            $this->logger->error('The submitted value %choice in the %name element is not allowed.', $message_arguments);
           }
         }
       }
@@ -364,8 +367,9 @@ protected function performRequiredValidation(&$elements, FormStateInterface &$fo
         $form_state->setValueForElement($elements, NULL);
       }
       elseif (!isset($options[$elements['#value']])) {
-        $form_state->setError($elements, $this->t('An illegal choice has been detected. Please contact the site administrator.'));
-        $this->logger->error('Illegal choice %choice in %name element.', ['%choice' => $elements['#value'], '%name' => empty($elements['#title']) ? $elements['#parents'][0] : $elements['#title']]);
+        $message_arguments = $message_arguments + ['%choice' => $elements['#value']];
+        $form_state->setError($elements, $this->t('The submitted value %choice in the %name element is not allowed.', $message_arguments));
+        $this->logger->error('The submitted value %choice in the %name element is not allowed.', $message_arguments);
       }
     }
   }
diff --git a/core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php b/core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php
index 17e020d8cc..d3d2e444d0 100644
--- a/core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php
+++ b/core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php
@@ -44,7 +44,7 @@ public function testBlockAddThemeSelector() {
     // Switch to a theme that doesn't contain the region selected above.
     $assert_session->selectExists('Theme')->selectOption('stark');
     $assert_session->assertWaitOnAjaxRequest();
-    $assert_session->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $assert_session->pageTextNotContains('The submitted value Pre-content in the Region element is not allowed.');
     $assert_session->optionExists('Region', '- Select -');
   }
 
diff --git a/core/modules/comment/tests/src/Functional/CommentFieldsTest.php b/core/modules/comment/tests/src/Functional/CommentFieldsTest.php
index 14b538bd4e..2356510283 100644
--- a/core/modules/comment/tests/src/Functional/CommentFieldsTest.php
+++ b/core/modules/comment/tests/src/Functional/CommentFieldsTest.php
@@ -165,7 +165,7 @@ public function testCommentFieldCreate() {
     $this->drupalGet('admin/config/people/accounts/fields/user.user.field_user_comment/storage');
     $this->submitForm($edit, 'Save field settings');
     // We should get an error message.
-    $this->assertSession()->pageTextContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextContains('The submitted value in the Comment type element is not allowed.');
 
     // Create a comment type for users.
     $bundle = CommentType::create([
@@ -183,7 +183,7 @@ public function testCommentFieldCreate() {
     $this->drupalGet('admin/config/people/accounts/fields/user.user.field_user_comment/storage');
     $this->submitForm($edit, 'Save field settings');
     // We shouldn't get an error message.
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('The submitted value in the Comment type element is not allowed.');
   }
 
   /**
diff --git a/core/modules/system/tests/src/Functional/Form/FormTest.php b/core/modules/system/tests/src/Functional/Form/FormTest.php
index 5599bf9dff..d0027d859f 100644
--- a/core/modules/system/tests/src/Functional/Form/FormTest.php
+++ b/core/modules/system/tests/src/Functional/Form/FormTest.php
@@ -143,7 +143,7 @@ public function testRequiredFields() {
             }
             if ($type == 'select') {
               // Select elements are going to have validation errors with empty
-              // input, since those are illegal choices. Just make sure the
+              // input, since those are not allowed choices. Just make sure the
               // error is not "field is required".
               $this->assertTrue((empty($errors[$element]) || strpos('field is required', (string) $errors[$element]) === FALSE), "Optional '$type' field '$element' is not treated as a required element");
             }
@@ -886,7 +886,7 @@ public function testInputForgery() {
     // an input forgery.
     // @see \Drupal\form_test\Form\FormTestInputForgeryForm::postRender
     $this->submitForm(['checkboxes[one]' => TRUE, 'checkboxes[two]' => TRUE], 'Submit');
-    $this->assertSession()->pageTextContains('An illegal choice has been detected.');
+    $this->assertSession()->pageTextContains('The submitted value FORGERY in the Checkboxes element is not allowed.');
   }
 
   /**
diff --git a/core/modules/system/tests/src/Functional/Form/ValidationTest.php b/core/modules/system/tests/src/Functional/Form/ValidationTest.php
index 0738c34b23..328fca30be 100644
--- a/core/modules/system/tests/src/Functional/Form/ValidationTest.php
+++ b/core/modules/system/tests/src/Functional/Form/ValidationTest.php
@@ -234,8 +234,10 @@ public function testCustomRequiredError() {
         $this->assertSession()->pageTextNotContains($form[$key]['#title'] . ' field is required.');
         $this->assertSession()->pageTextContains((string) $form[$key]['#form_test_required_error']);
       }
+      if (isset($form[$key]['#title'])) {
+        $this->assertSession()->pageTextNotContains('The submitted value in the ' . $form[$key]['#title'] . ' element is not allowed.');
+      }
     }
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
 
     // Verify that no custom validation error appears with valid values.
     $edit = [
@@ -255,8 +257,10 @@ public function testCustomRequiredError() {
         $this->assertSession()->pageTextNotContains($form[$key]['#title'] . ' field is required.');
         $this->assertSession()->pageTextNotContains((string) $form[$key]['#form_test_required_error']);
       }
+      if (isset($form[$key]['#title'])) {
+        $this->assertSession()->pageTextNotContains('The submitted value in the ' . $form[$key]['#title'] . ' element is not allowed.');
+      }
     }
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
   }
 
 }
diff --git a/core/modules/views/tests/src/Functional/Plugin/ExposedFormCheckboxesTest.php b/core/modules/views/tests/src/Functional/Plugin/ExposedFormCheckboxesTest.php
index e4b1b5417d..aa22353f1e 100644
--- a/core/modules/views/tests/src/Functional/Plugin/ExposedFormCheckboxesTest.php
+++ b/core/modules/views/tests/src/Functional/Plugin/ExposedFormCheckboxesTest.php
@@ -109,7 +109,7 @@ public function testExposedFormRenderCheckboxes() {
     // checked.
     $this->clickLink('Page 2');
     $this->assertSession()->elementsCount('xpath', "//div[contains(@class, 'views-row')]", 1);
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('The submitted value in the Type element is not allowed.');
   }
 
   /**
@@ -134,7 +134,7 @@ public function testExposedIsAllOfFilter() {
       ],
       'auto_create' => FALSE,
     ];
-    $this->createEntityReferenceField('node', 'article', $field_name, NULL, 'taxonomy_term', 'default', $handler_settings, FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED);
+    $this->createEntityReferenceField('node', 'article', $field_name, 'Reference Field', 'taxonomy_term', 'default', $handler_settings, FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED);
 
     // Add some test nodes.
     $this->createNode([
@@ -158,14 +158,14 @@ public function testExposedIsAllOfFilter() {
     // All rows are displayed by default on the first page when no options are
     // checked.
     $this->assertSession()->elementsCount('xpath', "//div[contains(@class, 'views-row')]", 8);
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('The submitted value in the Reference Field element is not allowed.');
 
     // Select one option and ensure we still have results.
     $tid = $this->terms[0]->id();
     $this->submitForm(["tid[$tid]" => $tid], 'Apply');
     // Ensure only nodes tagged with $tid are displayed.
     $this->assertSession()->elementsCount('xpath', "//div[contains(@class, 'views-row')]", 2);
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('The submitted value in the Reference Field element is not allowed.');
   }
 
 }
diff --git a/core/modules/views/tests/src/Functional/Plugin/FilterTest.php b/core/modules/views/tests/src/Functional/Plugin/FilterTest.php
index b153916439..0f6a158f56 100644
--- a/core/modules/views/tests/src/Functional/Plugin/FilterTest.php
+++ b/core/modules/views/tests/src/Functional/Plugin/FilterTest.php
@@ -173,7 +173,7 @@ public function testInOperatorSelectAllOptions() {
     $this->drupalGet('admin/structure/views/view/test_filter_in_operator_ui/edit/default');
     $this->submitForm([], 'Save');
     $this->submitForm([], 'Update preview');
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected.');
+    $this->assertSession()->pageTextNotContains('The submitted value "page" in the Type element is not allowed.');
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php b/core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php
index eccceecc59..74233ce3e5 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php
@@ -424,7 +424,7 @@ public function providerTestPerformRequiredValidation() {
           '#value' => 'baz',
           '#multiple' => FALSE,
         ],
-        'An illegal choice has been detected. Please contact the site administrator.',
+        'The submitted value <em class="placeholder">baz</em> in the <em class="placeholder">Test</em> element is not allowed.',
         TRUE,
       ],
       [
@@ -437,7 +437,7 @@ public function providerTestPerformRequiredValidation() {
           '#value' => ['baz'],
           '#multiple' => TRUE,
         ],
-        'An illegal choice has been detected. Please contact the site administrator.',
+        'The submitted value <em class="placeholder">0</em> in the <em class="placeholder">Test</em> element is not allowed.',
         TRUE,
       ],
       [
@@ -450,7 +450,7 @@ public function providerTestPerformRequiredValidation() {
           '#value' => ['baz'],
           '#multiple' => TRUE,
         ],
-        'An illegal choice has been detected. Please contact the site administrator.',
+        'The submitted value <em class="placeholder">baz</em> in the <em class="placeholder">Test</em> element is not allowed.',
         TRUE,
       ],
       [