Custom error

It's in the webconfig, not sitecore config

System.Web.Configuration.CustomErrorsMode.Off (On, RemoteOnly)

 <!--  CUSTOM ERROR MESSAGES
          Set customError mode values to control the display of user-friendly
          error messages to users instead of error details (including a stack trace):

          "On" Always display custom (friendly) messages
          "Off" Always display detailed ASP.NET error information.
          "RemoteOnly" Display custom (friendly) messages only to users not running
          on the local Web server. This setting is recommended for security purposes, so
          that you do not display application detail information to remote clients.
    -->