Web.Config

ErrorController.cs

    public class ErrorController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }

        public ActionResult NotFound(string aspxerrorpath)
        {
            return RedirectToAction("Index", "Home");
        }

    }

gistfile1.txt

    <customErrors mode="RemoteOnly" defaultRedirect="~/erro">
      <error statusCode="404" redirect="~/erro/naoencontrado" />
    </customErrors>

Global.asax.cs

public static void RegisterGlobalFilters(GlobalFilterCollection filters)
{
     filters.Add(new HandleErrorAttribute());
}

Minha versão: https://gist.github.com/4131799