-`proxy-invalid.pem` certificate is used. It's easy to spot in the browser
-because it uses an invalid hostname ("invalid") and is self-signed.
+`proxy-invalid.pem` certificate is used to send a 500 error message to the
+client. The connection to the server is closed so there's no chance that any
+client data is sent to the (possible) evil server. The invalide certificate is
+also easy to spot in the browser because it uses an invalid hostname
+("invalid") and is self-signed.