Problem with 404 and custom error page

Hi
asp.net 3.5
I have this settings in web.config
<customErrors mode="remoteOnly" defaultRedirect="Error.htm">
     <error statusCode="404" redirect="NoSuchPage.aspx" />
</customErrors> 
To my surprise I see that someone have created a link to my site like this:
http://www.mydomain/default.aspx%33.
This page gives a 404 http response code. I was sure that my customError settings in web.config would catch the error and open NoSuchPage.aspx instead, but I was wrong. Instead it gave a traditional 404 error page.
In addition I tested if I tryed to navigate to a page which I knows does not exists in the site, I tryed to navigate to http://www.mydomain.com/default4.aspx this gave a blank page, no 404 error. This defalt4.aspx page does not exists in th ...