Hmm jeg har lige læst at man ikke kan fange fejlen fra ens egen fejlside:
"There is a problem here with this solution. Once the redirect is done, your error information is no longer available on the redirected page. This is because IIS (via the .net framework) performs a plain old GET request to the error page and does not do a "Server.Transfer" like the built-in IIS error handling does.
The only information available to you at this time is the URL that caused this error to be raised. This is located on the querystring as "aspxerrorpath":
http://localhost/ErrorHandling/error500.aspx?aspxerrorpath=/ErrorHandling/WebForm1.aspx. The only places this information is available is the two methods described above."
Så hvis du vil logge din fejl, må du gøre det i Application_Error event'et i global.asax
Eneste problem er desværre så at man ikke kender fejlen i ens error-side, og derfor ikke kan vise andet end at "der skete en fejl".