I changed my code from
<?php
if(!in_array('event',$node_types)) array_unshift($node_types,'event', 'volunteer_timeslots');
if ((user_access('create events') || user_access('edit own events') || user_access('create volunteer events') || user_access('edit own volunteer events')) && in_array($node_type, $node_types)) {
?>to
<?php
if(!in_array('event',$node_types)) array_unshift($node_types,'event');
if ((user_access('create events') || user_access('edit own events') || user_access('create volunteer events') || user_access('edit own volunteer events')) && in_array($node_type, $node_types)) {
?>and I'm still getting the warning message.