I have used current environment for debugging error message, by setting CustomError to Off and CallStack to true. I have fixed the issue and now try to set back this CustomError to On and CallStack back to false. However, the friendly SharePoint error page never come out, it shows 500 Internal Server Error text instead. It seems that whenever I set stacktrace="false", it will show 500 Internal Server Error. There are a few errors that should show in SharePoint friendly error page, like "You are not allowed to respond again to this survey", but it shows 500 Internal Server Error page. Any idea how to reset it back?