As drewm himself said this is due to the subsequent redirect after the POST to the script has in fact succeeded. (I might have added this as a comment to his answer but you need 50 reputation to comment and I'm new round here - daft rule IMHO)
BUT it also applies if you're trying to redirect to a page, not just a directory - at least it did for me. I was trying to redirect to /thankyou.html. What fixes this is using an absolute URL, i.e. http://example.com/thankyou.html