I noticed entries like these in my error logs:

[24-Jan-2010 18:52:32] PHP Fatal error:  Uncaught exception 'ErrorException' with message 'Undefined index:  assoc_type' in /foo/bar/Zend/OpenId/Consumer.php:635

The problem is this line:

if ($params['openid.assoc_type'] != $ret['assoc_type']) {

If assoc_type isn't set, it is consequently used in this line, thus raising a notice. This is bad, since I convert all notices to exceptions. As the application is already in an error state here, the impact is minor though.