If there was no description the clean_preprocess_fieldset was adding an array to the string displayed. I'm not sure exactly what as intended, but I'm guessing it was to initialize the variable if it was not yet set (but of course it's a string not an arrary), and the append the description etc.

Something like:-

Index: template.php
===================================================================
RCS file: /cvs/drupal-contrib/contributions/themes/clean/template.php,v
retrieving revision 1.1.2.7.2.17.2.43
diff -u -p -r1.1.2.7.2.17.2.43 template.php
--- template.php	10 Sep 2010 12:27:31 -0000	1.1.2.7.2.17.2.43
+++ template.php	16 Oct 2010 11:21:51 -0000
@@ -330,10 +330,10 @@ function clean_preprocess_fieldset(&$var
     $value = $element['#value'];
   }
   if (empty($variables['content'])) {
-    $variables['content'] = array();
+    $variables['content'] = '';
   }
   if (!empty($description)) {
-    $variables['content'] = $description;
+    $variables['content'] .= $description;
   }
   if (!empty($children)) {
     $variables['content'] .= $children;