I think everyone knows about the <customErrors> tag in the web.config to enable error details to be displayed by the Runtime Error page, which you usually get when your aspx markup and related code-behind has a problem.

 

Normally this is set in the Virtual directory of the Web application along with another version of the web.config file in the Template/Layouts folder of the SharePoint HIVE.

 

But if you are developing Admin pages you need to set the RemoteOnly setting in the web.config that is found in the Template/Admin folder of the SharePoint HIVE.