This example will cover creating a custom error page for 404 Page Not Found and 500 Server Error. You can extend this code to capture any error code you need to.

Web.Config

If you are using IIS7 and above, ignore the <CustomError.. node and use <httpErrors... instead.

Add in the following in the system.webServer node:

<httpErrors errorMode="Custom" existingResponse="Replace">
    <remove statusCode="404" />
    <remove statusCode="500" />
    <error statusCode="404" path="/error/notfound" responseMode="ExecuteURL" />
    <error statusCode="500" path="/error/servererror" responseMode="ExecuteURL" />
 </httpErrors>

This tells the site to direct any 404 errors to ~/error/notfound and any 500 error to ~/error/servererror. It will also preserve your requested URL (think transfer rather than redirect) so the user will never see the ~/error/... page URL.

Next, you need a new Error controller so...

public class ErrorController : Controller
{
    public ActionResult servererror()
    {
        Response.TrySkipIisCustomErrors = true;
        Response.StatusCode = (int)HttpStatusCode.InternalServerError;
        return View();
    }

    public ActionResult notfound()
    {
        Response.TrySkipIisCustomErrors = true;
        Response.StatusCode = (int)HttpStatusCode.NotFound;
        return View();
    }

} 

The key thing to note here is the Response.TrySkipIisCustomErrors = true;. This will bypass IIS and force your error page through.

Lastly, create the corresponding NotFound and ServerError Views and style them up so it's all nice and seamless with your site's design.

Hey presto - custom error pages.