public class CustomErrorHandlingModule : IHttpModule
{
    #region Implementation of IHttpModule

    public void Init(HttpApplication context)
    {
        context.Error += (sender, e) =>
            OnError(new HttpContextWrapper(((HttpApplication)sender).Context));
    }

    public void Dispose()
    {}

    public void OnError(HttpContextBase context)
    {
        // Determine error resource to display, based on HttpStatus code, etc.
        // For brevity, i'll hardcode it for this SO question.
        const string errorPage = @"/Error/NotFound";

        // Now somehow execute the correct controller for that route.
        // Return the html response.
    }
}

public void OnError(HttpContextBase context)
{
    context.ClearError();
    context.Response.StatusCode = 404;

    var rd = new RouteData();
    rd.Values["controller"] = "error";
    rd.Values["action"] = "notfound";
    IController controller = new ErrorController();
    var rc = new RequestContext(context, rd);
    controller.Execute(rc);
}

System.Web.HttpContext.Current.Response.Redirect("/Error/NotFound");

// MVC 3 running on IIS 7+
    if (HttpRuntime.UsingIntegratedPipeline)
    {
        context.Server.TransferRequest(url, true);
    }
    else
    {
        // Pre MVC 3
        context.RewritePath(url, false);

        IHttpHandler httpHandler = new MvcHttpHandler();
        httpHandler.ProcessRequest(httpContext);
    }

HttpApplication app = (HttpApplication) context.Application;
app.CompleteRequest();;