Web.config customErrors element

In de web.config of a (web) application it is possible te configure the customErrors element. Mostly for development I want to see the details of the errors so I can fix these quickly. But when the application runs for end-users I want to be informed about these errors and the users sees a friendly message "there is something wrong, this is registrerd".

A More Useful 404

On A List Apart you can read some advices to create A More Useful 404.