ASP.NET customErrors Tutorial
You can use any file as a 404 error file in the defaultRedirect attribute value, but I suggest you use a new web form file called 404.aspx, or a generic handler .
http://www.dotnetperls.com/customerrors

web config - Asp.net - <customErrors mode="Off"/> error when trying ...
Notes: The current error page you are seeing can be replaced by a custom error page by >modifying the "defaultRedirect" attribute of the .
http://stackoverflow.com/questions/4363941/asp-net-customerrors-mode-off-error-when-trying-to-access-working-webpage

asp.net - Web.config isn't showing errors remotely even with ...
This <customErrors> tag should then have its "mode" attribute set to "Off". . page by modifying the "defaultRedirect" attribute of the application's .
http://stackoverflow.com/questions/5043339/web-config-isnt-showing-errors-remotely-even-with-customerrors-off-on-ubuntu

Custom Error Pages in ASP.NET
<customErrors mode="RemoteOnly" defaultRedirect="error/default.aspx" />. The attribute mode enables or disables custom errors. When the custom error mode .
http://riderdesign.com/articles/displayarticle.aspx?articleid=1