500 server error[
^].
If this is happening on your web application, then you need to check your error logs, or if you haven't got any, you need to implement logging.
If this is someone else's site, then they likely have logging and they will see that the error occurred. If they were foolish enough to publish a site without logging, then they may have a contact us page or something like that where you can send them information regarding the error you saw.