diff --git a/modules/system/system.admin.inc b/modules/system/system.admin.inc
index 3313f5a..e72075a 100644
--- a/modules/system/system.admin.inc
+++ b/modules/system/system.admin.inc
@@ -2181,24 +2181,45 @@ function system_site_maintenance_mode() {
  * @see system_settings_form()
  */
 function system_clean_url_settings($form, &$form_state) {
-  global $base_url;
-
-  // When accessing this form using a non-clean URL, allow a re-check to make
-  // sure clean URLs can be disabled at all times.
   $available = FALSE;
-  if (strpos(request_uri(), '?q=') === FALSE || !empty($_SESSION['clean_url'])) {
+  $conflict = FALSE;
+
+  // If the request URI is a clean URL, clean URLs must be available.
+  // Otherwise, run a test.
+  if (strpos(request_uri(), '?q=') === FALSE && strpos(request_uri(), '&q=') === FALSE) {
     $available = TRUE;
   }
   else {
-    $request = drupal_http_request($base_url . '/admin/config/search/clean-urls/check');
+    $request = drupal_http_request($GLOBALS['base_url'] . '/admin/config/search/clean-urls/check');
+    // If the request returns HTTP 200, clean URLs are available.
     if (isset($request->code) && $request->code == 200) {
       $available = TRUE;
+      // If the user started the clean URL test, provide explicit feedback.
+      if (isset($form_state['input']['clean_url_test_execute'])) {
+        drupal_set_message(t('The clean URL test passed.'));
+      }
+    }
+    else {
+      // If the test failed while clean URLs are enabled, make sure clean URLs
+      // can be disabled.
+      if (variable_get('clean_url', 0)) {
+        $conflict = TRUE;
+        // Warn the user of a conflicting situation, unless after processing
+        // a submitted form.
+        if (!isset($form_state['input']['op'])) {
+          drupal_set_message(t('Clean URLs are enabled, but the clean URL test failed. Uncheck the box below to disable clean URLs.'), 'warning');
+        }
+      }
+      // If the user started the clean URL test, provide explicit feedback.
+      elseif (isset($form_state['input']['clean_url_test_execute'])) {
+        drupal_set_message(t('The clean URL test failed.'), 'warning');
+      }
     }
   }
 
-  if ($available) {
-    $_SESSION['clean_url'] = TRUE;
-
+  // Show the enable/disable form if clean URLs are available or if the user
+  // must be able to resolve a conflicting setting.
+  if ($available || $conflict) {
     $form['clean_url'] = array(
       '#type' => 'checkbox',
       '#title' => t('Enable clean URLs'),
@@ -2206,18 +2227,37 @@ function system_clean_url_settings($form, &$form_state) {
       '#description' => t('Use URLs like <code>example.com/user</code> instead of <code>example.com/?q=user</code>.'),
     );
     $form = system_settings_form($form);
+    if ($conflict) {
+      // $form_state['redirect'] needs to be set to the non-clean URL,
+      // otherwise the setting is not saved.
+      $form_state['redirect'] = url('', array('query' => array('q' => '/admin/config/search/clean-urls')));
+    }
   }
+  // Show the clean URLs test form.
   else {
     drupal_add_js(drupal_get_path('module', 'system') . '/system.js');
 
-    $form_state['redirect'] = $base_url . '/admin/config/search/clean-urls';
+    $form_state['redirect'] = url('admin/config/search/clean-urls');
     $form['clean_url_description'] = array(
       '#type' => 'markup',
-      '#markup' => '<p>' . t('Use URLs like <code>example.com/user</code> instead of <code>example.com/?q=user</code>.') . ' ' . t('If you are directed to a <em>Page not found (404)</em> error after testing for clean URLs, see the <a href="@handbook">online handbook</a>.', array('@handbook' => 'http://drupal.org/node/15365')) . '</p>',
+      '#markup' => '<p>' . t('Use URLs like <code>example.com/user</code> instead of <code>example.com/?q=user</code>.'),
     );
-    $form['clean_url_test'] = array(
-      '#type' => 'submit',
-      '#value' => t('Run the clean URL test'),
+    // Explain why the user is seeing this page and tell him what to expect
+    // after clicking the 'Run the clean URL test' button.
+    $form['clean_url_test_result'] = array(
+      '#type' => 'markup',
+      '#markup' => '<p>' . t('Clean URLs cannot be enabled. If you are directed to this page or to a <em>Page not found (404)</em> error after testing for clean URLs, see the <a href="@handbook">online handbook</a>.', array('@handbook' => 'http://drupal.org/node/15365')) . '</p>',
+    );
+    $form['actions'] = array(
+      '#type' => 'actions',
+      'clean_url_test' => array(
+        '#type' => 'submit',
+        '#value' => t('Run the clean URL test'),
+      ),
+    );
+    $form['clean_url_test_execute'] = array(
+      '#type' => 'hidden',
+      '#value' => 1,
     );
   }