diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -405,10 +405,7 @@
 
   // Use a default value if $message is not set.
   if (empty($message)) {
-    // The exception message is run through
-    // \Drupal\Component\Utility\SafeMarkup::checkPlain() by
-    // \Drupal\Core\Utility\Error:decodeException().
-    $message = '%type: !message in %function (line %line of %file).';
+    $message = '%type: @message in %function (line %line of %file).';
   }
 
   if ($link) {
@@ -488,7 +485,10 @@
 
   // Use a default value if $message is not set.
   if (empty($message)) {
-    $message = '%type: @message in %function (line %line of %file).';
+    // The exception message is run through
+    // \Drupal\Component\Utility\SafeMarkup::checkPlain() by
+    // \Drupal\Core\Utility\Error:decodeException().
+    $message = '%type: !message in %function (line %line of %file).';
   }
 
   if ($link) {
diff -u b/core/modules/migrate/src/MigrateExecutable.php b/core/modules/migrate/src/MigrateExecutable.php
--- b/core/modules/migrate/src/MigrateExecutable.php
+++ b/core/modules/migrate/src/MigrateExecutable.php
@@ -363,7 +363,7 @@
    */
   protected function handleException(\Exception $exception, $save = TRUE) {
     $result = Error::decodeException($exception);
-    $message = $result['@message'] . ' (' . $result['%file'] . ':' . $result['%line'] . ')';
+    $message = $result['!message'] . ' (' . $result['%file'] . ':' . $result['%line'] . ')';
     if ($save) {
       $this->saveMessage($message);
     }
@@ -405,7 +405,7 @@
    */
   protected function handleException(\Exception $exception, $save = TRUE) {
     $result = Error::decodeException($exception);
-    $message = $result['!message'] . ' (' . $result['%file'] . ':' . $result['%line'] . ')';
+    $message = $result['@message'] . ' (' . $result['%file'] . ':' . $result['%line'] . ')';
     if ($save) {
       $this->saveMessage($message);
     }
diff -u b/core/modules/system/src/Tests/System/ErrorHandlerTest.php b/core/modules/system/src/Tests/System/ErrorHandlerTest.php
--- b/core/modules/system/src/Tests/System/ErrorHandlerTest.php
+++ b/core/modules/system/src/Tests/System/ErrorHandlerTest.php
@@ -44,7 +44,7 @@
     );
     $error_user_notice = array(
       '%type' => 'User warning',
-      '@message' => 'Drupal is <awesome>',
+      '@message' => 'Drupal is awesome & amazing',
       '%function' => 'Drupal\error_test\Controller\ErrorTestController->generateWarnings()',
       '%file' => drupal_get_path('module', 'error_test') . '/error_test.module',
     );
diff -u b/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php b/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php
--- b/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php
+++ b/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php
@@ -51,8 +51,9 @@
     $monkey_love = $bananas;
     // This will generate a warning.
     $awesomely_big = 1/0;
-    // This will generate a user error.
-    trigger_error("Drupal is <awesome>", E_USER_WARNING);
+    // This will generate a user error. Includes an ampersand so we can
+    // check for any escaping.
+    trigger_error("Drupal is awesome & amazing", E_USER_WARNING);
     return [];
   }
 
only in patch2:
unchanged:
--- a/core/modules/system/src/Tests/Common/SimpleTestErrorCollectorTest.php
+++ b/core/modules/system/src/Tests/Common/SimpleTestErrorCollectorTest.php
@@ -44,7 +44,7 @@ function testErrorCollect() {
     if (count($this->collectedErrors) == 3) {
       $this->assertError($this->collectedErrors[0], 'Notice', 'Drupal\error_test\Controller\ErrorTestController->generateWarnings()', 'ErrorTestController.php', 'Undefined variable: bananas');
       $this->assertError($this->collectedErrors[1], 'Warning', 'Drupal\error_test\Controller\ErrorTestController->generateWarnings()', 'ErrorTestController.php', 'Division by zero');
-      $this->assertError($this->collectedErrors[2], 'User warning', 'Drupal\error_test\Controller\ErrorTestController->generateWarnings()', 'ErrorTestController.php', 'Drupal is awesome');
+      $this->assertError($this->collectedErrors[2], 'User warning', 'Drupal\error_test\Controller\ErrorTestController->generateWarnings()', 'ErrorTestController.php', 'Drupal is awesome & amazing');
     }
     else {
       // Give back the errors to the log report.
only in patch2:
unchanged:
--- a/core/modules/system/tests/modules/error_service_test/src/Logger/TestLog.php
+++ b/core/modules/system/tests/modules/error_service_test/src/Logger/TestLog.php
@@ -24,7 +24,7 @@ class TestLog implements LoggerInterface {
   public function log($level, $message, array $context = array()) {
     $trigger = [
       '%type' => 'Exception',
-      '!message' => 'Deforestation',
+      '@message' => 'Deforestation',
       '%function' => 'Drupal\error_service_test\MonkeysInTheControlRoom->handle()',
       'severity_level' => 3,
       'channel' => 'php',