diff -u b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
--- b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
@@ -87,7 +87,7 @@
    * {@inheritdoc}
    */
   public static function open(array &$connection_options = array()) {
-    if (isset($connection_options['#dsn_utf8_fallback']) && $connection_options['#dsn_utf8_fallback'] === TRUE) {
+    if (isset($connection_options['_dsn_utf8_fallback']) && $connection_options['_dsn_utf8_fallback'] === TRUE) {
       // Only used during the installer version check, as a fallback from utf8mb4.
       $charset = 'utf8';
     }
diff -u b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
--- b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
@@ -77,7 +77,7 @@
           $info_copy = $info;
           // Set a flag to fall back to utf8. Note: this flag should only be
           // used here and is for internal use only.
-          $info_copy['default']['#dsn_utf8_fallback'] = TRUE;
+          $info_copy['default']['_dsn_utf8_fallback'] = TRUE;
           // In order to change the Database::$databaseInfo array, we need to
           // remove the active connection, then re-add it with the new info.
           Database::removeConnection('default');
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Database/Database.php
+++ b/core/lib/Drupal/Core/Database/Database.php
@@ -298,6 +298,28 @@
   }
 
   /**
+   * Validates that database settings array does not contain any options
+   * meant for internal use only.
+   *
+   * @param array $databases
+   *   A multi-dimensional array specifying database connection parameters, as
+   *   defined in settings.php.
+   *
+   * @throws \Drupal\Core\Database\InvalidDatabaseSettingException
+   */
+  final public static function validateDatabaseSettings(array $databases) {
+    foreach ($databases as $key => $targets) {
+      foreach ($targets as $target => $info) {
+        foreach (array_keys($info) as $settings_key) {
+          if (0 === strpos($settings_key, '_')) {
+            throw new InvalidDatabaseSettingException('The ' . $settings_key . ' database setting is for internal use only.');
+          }
+        }
+      }
+    }
+  }
+
+  /**
    * Rename a connection and its corresponding connection information.
    *
    * @param string $old_key
only in patch2:
unchanged:
--- /dev/null
+++ b/core/lib/Drupal/Core/Database/InvalidDatabaseSettingException.php
@@ -0,0 +1,12 @@
+<?php
+/**
+ * @file
+ * Contains \Drupal\Core\Database\InvalidDatabaseSettingException.
+ */
+
+namespace Drupal\Core\Database;
+
+/**
+ * Exception thrown if a database setting in settings.php is invalid.
+ */
+class InvalidDatabaseSettingException extends \RuntimeException {}
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Site/Settings.php
+++ b/core/lib/Drupal/Core/Site/Settings.php
@@ -122,6 +122,7 @@ public static function initialize($app_root, $site_path, &$class_loader) {
     }
 
     // Initialize Database.
+    Database::validateDatabaseSettings($databases);
     Database::setMultipleConnectionInfo($databases);
 
     // Initialize Settings.