Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.585.2.50
diff -u -p -r1.585.2.50 system.module
--- modules/system/system.module	4 Mar 2010 00:25:13 -0000	1.585.2.50
+++ modules/system/system.module	11 Apr 2010 14:01:00 -0000
@@ -920,7 +920,13 @@ function _system_theme_data() {
     $themes_info = $themes;
   }
 
-  return $themes_info;
+  // To avoid side effects, we don't return the original objects.
+  $themes_info_cloned = array();
+  foreach ($themes_info as $key => $theme) {
+    $themes_info_cloned[$key] = drupal_clone($theme);
+  }
+
+  return $themes_info_cloned;
 }
 
 /**