Deployment Element – force the ‘debug’ flag in the web.config to be false

To disables ASP.NET certain configuration settings such as trace output, custom errors, and debug capabilities.

Set retail=”true” in your machine.config in the production server

<configuration>

<system.web>

<deployment retail=”true”/>

</system.web>

</configuration>

It will force the ‘debug’ flag in the web.config to be false, disable page output tracing, and�
also force the custom error page to be shown to remote users rather than the actual exception or error message

Note: Important

This value can only be set at the machine level, not at the application level, so do it only Production server.

REF MSDN