--- system.module	Thu Jan 15 10:14:12 1970
+++ system.module	Thu Jan 15 10:14:12 1970
@@ -920,7 +920,19 @@
     $themes_info = $themes;
   }
 
-  return $themes_info;
+  // To avoid side effects, we don't return the original objects
+  if (version_compare(PHP_VERSION, '5.0.0', '>')) {
+    $themes_info_cloned = array();
+    // only PHP 5+ knows the "clone" keyword.
+    // PHP 4 clones objects by itself, unless they are by reference variables.
+    foreach ($themes_info as $key => $theme) {
+      $themes_info_cloned[$key] = clone $theme;
+    }
+    return $themes_info_cloned;
+  }
+  else {
+    return $themes_info;
+  }
 }
 
 /**