I've a little mantra at work: the lower the frequency, the more trouble it is. GPIO (DC) wiring is notoriously tricky to get right. 10Gbps #Ethernet? Just works.

Except, Reader, today, 10G Ethernet did NOT "just work".

Some cursed combination of fibre faults, picky equipment and bad luck has meant for a deeply frustrating afternoon of not fixing something.

Sigh.