To turn off custom errors, you should add these lines in to your Web.config:
The <customErrors> section enables configuration
of what to do if/when an unhandled error occurs
during the execution of a request. Specifically,
it enables developers to configure html error pages
to be displayed in place of a error stack trace.
-->
<customErrors mode="Off" />
If you already have a customErrors declaration, or any error page declarations you'll need to remove these lines. Turning off custom errors will help you diagnose most ASP.net coding issues.
