[RedDevil]

In the early 90's I used to work on the computer department of a large bank, where one of the favorite stories among operators was the Christmas Power Outage.

The bank had a centralized UPS for all the critical components of the mainframe. The CPUs, the hard disk units, the tape decks, the printers, even the invoice readers were all on the system. The whole system was in a separate building which was built almost like a bunker, with a double security zone and all the hardware in the inner zone with no windows.

What wasn't on the system were general lighting, the non-essential terminals and monitors (pre-PC), coffee machines, etc. But our grid was fairly reliable so it all worked for years without a hitch.

Then one day before Christmas the power went down.
And everything went pitch black, due to the bunker structure. All the lighting, all the screens, even the master consoles. The only light came from the power lights on the computer hardware which kept running and buzzing as usual, even without any control or monitoring.

And one tiny plastic Christmas tree, right next to the stricken Master Consoles...

It did not take my colleagues long to plug the Master Consoles into the correct power sockets. The Christmas tree saved the day!

It did not take too long for the main power to come back on so they could even use the powered doors again.
Some changes to the setup were made afterwards