The customer was a Bank. One the biggest banks in Europe. They just bought (aka "rescued") another bank somewhere in Europe and they wanted to lay dark fiber

The customer was a Bank. One the biggest banks in Europe. They just bought (aka "rescued") another bank somewhere in Europe and they wanted to lay dark fiber for datacenter synchronization. And they wanted us to manage that network. After several weeks laying fiber, and setting all connections we started the network manager on the operations center... and couldn't see any device, except the "secondary" network manager in customer premises wich could see everything. Turns out the "security" people at the simply put a Firewall between the NOC and the network and didn't allow any traffic to go from the NOC to the fiber optic devices. We asked them to give us access but they didn't have IPv4 addressing compatible with ours. Whay? Because the bank used all IPv4 address space internally. All private IP adressing was already alocated and the NAT ("public") they could use colided with other customers equipment. Finally I had to build a ugly "semi-isolated" network island between the bank, the noc, and some virtualized workstations that could "see" (natted) the customer devices (wich we were paid to manage) and the NOC workstations. On the plus side, that junk made me write the best documentation I've ever made because just in case something broke in the middle of the night.