When I have custom error turned on for my site, and then try www.domain.com/robots.txt..// , it errors out to an error page which says 

Server Error in '/' Application.

Runtime Error

Description: An exception occurred while processing your request. Additionally, another exception occurred while executing the custom error page for the first exception. The request has been terminated. 

How can debug this error or fix it so that the standard error page shows up?

Thanks.

- Dhillip