If you are using a custom error handler (if not, you probably will be soon), you need to register it with the front controller:
$front = Zend_Controller_Front::getInstance();
$front->registerPlugin(new Zend_Controller_Plugin_ErrorHandler(array(
'module' => 'error',
'controller' => 'error',
'action' => 'error'
)));
This means you have a module, a controller and action all called 'error'. If you don't have a custom error controller yet, google it or have a read through http://www.thedeveloperday.com/custom-profiler-for-live-environments/