Rendering field cases unexpected error
You need to load the node to render it if the page you are viewing is other than node page... In node page page.tpl.php contains $node object.. See page.tpl.php documentation for more details...


  $node: The node object, if there is an automatically-loaded node
  associated with the page, and the node ID is the second argument in
  the page's path (e.g. node/12345 and node/12345/revisions, but not
  comment/reply/12345).


If other than node page you need to load the specific node from which you want to render the field in your login/registration/profile pages.. You can override page.tpl.php for specific pages.. For more details see Template suggestions..

To load the node specifically you can use node_load API function...

$node = node_load($nid); // Where $nid is node id