They have an office in London and another in Milan.
They could sit in front of a machine in London, and ping the London router. They could ping the Milan router. But they couldn't ping anything else on the Milan LAN.
Cisco engineers -- engineers from Cisco -- have been out in Milan all last night, swapped the router, swapped the switch swapped the wires, couldn't fix it.
So I arrived at the London office, opened up the laptop, and started to play.
Fixed it very quickly -- all stuff you'll have covered in your ICND course.
And my client thinks I'm a genius.
(router autonegotiating to 100Mbps, full duplex -- switch autonegotiating to 100Mbps, half duplex)