The <customErrors> section lets you configure what happens 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.
To turn off custom errors, simply add these lines into your web.config file.
<customErrors mode="Off" />
If you already have a custom Errors 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.