A strange thing I noticed is My404Page was not working properly on Firefox, I then tested it on IE and it worked.
When I browse my website it shows the HTML tags on the page.
When I test the site using http://dewani.net/123.aspx My custom 404 page showed up correctly
When I test the site using http://dewani.net/123 My custom 404 page showed up HTML tags.
When I digging and goggling I found it