ajax redirect to exception/error pages

Spot on again Dave!.

I find that
var url= "http://localhost:8080/errors/html/error_404";
window.location = url;

is working great but
var url= "http://localhost:8080/errors/html/production";
window.location = url;

Does not return back the production "generic error" page.
Makes me wonder if my 404 is actually working or failing and redirecting me to a 404 anyway!

Any tips on this one?