DRY approach with different request type

Assuming you always want to reject AJAX requests this should work.
It responds to any AJAX request with an "Internal Server Error"

PHP Code:

function check_failed()
{
 if(is_ajax_request())
 {
 $this->output
 ->set_status_header(500)
 ->set_header("Content-Type: text/plain")
 ->set_output("Internal Server Error")
 ->_display();
 exit();
 }
 elseif(check_condition_failed)
 {
 redirect(); // or show_404
 }
} 


And you will be much more DRY by putting this in a "base" controller that will be extended by the "Class#" controllers.