I've been constantly seeing errors from apt-get update being unable to connect to
apt.puppetlabs.com for months.
It is very confusing because apt tries the ipv4 address first. It times out. Then, it tries the ipv6 address which for must of us results in a network unreachable; however, it doesn't display an error about the timeout and only display's the network unreachable from the ipv6 attempt. Annoying!