Suppose you configured custom 404 page in web.config file in the customErrors section. So whenever user requests non-existent aspx page, ASP.NET run-time returns well formatted message to the user.
Also you have a page that shows articles from a database according to article ID passed in the url (for example: article.aspx?id=345). But if user passes article ID that doesn't exists the page must return code 404 (Not Found) and show the custom 404 page like in the previous situation.
Fortunately you don't need to parse the customErrors section to get name of the custom 404 page. Just throw HttpException:
throw new HttpException(404, "Article not found");
ASP.NET run-time will catch the exception and will redirect to the custom 404.