"At Let's Encrypt, we are seeing clients in the wild that continue polling their challenges long after those challenges have expired and started serving 404." The 4xx response code errors are client errors and should not be retried. Fixes golang/go#24145 Change-Id: I012c584fc4defd3a0d64a653860c35705c5c6653 Reviewed-on: https://go-review.googlesource.com/97695 Run-TryBot: Alex Vaghin <ddos@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>