Library path problem

[eluser]Wayne Smallman[/eluser]
Hi guys!

I've successfully loaded a library that's within the application/library folder, but this fails to load on the live website:

Code:

$uri_segment['client_id'] = $this->uri->segment(3);

        if (isset($uri_segment['client_id'])
            && ($uri_segment['client_id'] > 0)):

            if (file_exists(APPPATH . 'libraries/customers/' . $uri_segment['client_id'] . '/processing.php')):

                $this->load->library('customers/' . $uri_segment['client_id'] . '/processing');

            endif;

        endif;

The APPPATH constant is identical in each instance.

Specifically, it's the file_exists() function that's failing, and when I try visiting the file or file path, rather than get the obligatory 403 error, I'm getting a 404.

Any ideas what's happening?