Signature is not valid (pkg)

I just installed FreeBSD 10.0-RELEASE onto a machine that has previously worked fine with FreeBSD 10-RELEASE (new hard drive). Apologies if this is a noob question, I've just started trying out FreeBSD, I've been using various *nix for several years though. I tried to do pkg update to make it install the pkg tool. It said:
Code:
Verifying signature with trusted certificate pkg.freebsd.org.2013102301 ... failed
Signature is not valid

I've searched both the forum and Google, and I'm still not really sure what to do.
 
Fascinating. It just suddenly started working without so much as a reboot. I'd still welcome an explanation of what was the matter/why it works now, but at least it works now.
 
Looks like this is a recurring problem. I just observed a similar case right after updating from 9.2 to 10.0.

Here is what I got first:

Code:
# pkg
The package management tool is not yet installed on your system.
Do you want to fetch and install it now? [y/N]: y
Bootstrapping pkg from pkg+http://pkg.FreeBSD.org/freebsd:10:x86:64/latest, please wait...
pkg: error:04091068:lib(4):func(145):reason(104)
Verifying signature with trusted certificate pkg.freebsd.org.2013102301... failed
Signature is not valid

# cat /etc/pkg/FreeBSD.conf
# $FreeBSD: release/10.0.0/etc/pkg/FreeBSD.conf 258710 2013-11-28 14:24:26Z gjb $
FreeBSD: {
  url: "pkg+http://pkg.FreeBSD.org/${ABI}/latest",
  mirror_type: "srv",
  signature_type: "fingerprints",
  fingerprints: "/usr/share/keys/pkg",
  enabled: yes
}

Then, I started googling, found this post, and after a while, tried again. And it worked out of the blue without a reboot.

Code:
# pkg
The package management tool is not yet installed on your system.
Do you want to fetch and install it now? [y/N]: y
Bootstrapping pkg from pkg+http://pkg.FreeBSD.org/freebsd:10:x86:64/latest, please wait...
Verifying signature with trusted certificate pkg.freebsd.org.2013102301... done
Installing pkg-1.3.7: 100%
Message for pkg-1.3.7:
 If you are upgrading from the old package format, first run:
 
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