public class SomeController : Controller
{
    [HttpPost]
    public async Task<HttpResponseMessage> Foo()
    {
        var response = new HttpResponseMessage(HttpStatusCode.BadRequest);
        response.Content = new StringContent("Naughty");

        return response;
    }
}

return new HttpStatusCodeResult(HttpStatusCode.BadRequest, "naughty");

public class SomeController : Controller
{
    [HttpPost]
    public async Task<ActionResult> Foo()
    {
        Response.StatusCode = 400;
        return Content("Naughty");
    }
}

<system.webServer>
    <httpErrors existingResponse="PassThrough"/>
 </system.webServer>

// GET: Student/Details/5
public ActionResult Details(int? id)
{
    if (id == null)
    {
        return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
    }
    Student student = db.Students.Find(id);
    if (student == null)
    {
        return HttpNotFound();
    }
    return View(student);
}