Ah, reminds me of my previous jobs.
1) in the army testing some new communications equipment. Change over between generators meant relying on the UPS. This failed within seconds as someone hadn't wired up all the batteries.
2) second job looking after a large number of linux servers on my last night shift and the internet stopped working. Identified that the comms room was dark and that we'd lost power. Eventually someone located a bug in the UPS firmware meaning that the redundant supplies, generators and UPS's we had were all useless. I wouldn't have known until the hourly checks came round if the internet hadn't stopped!
Power failure, tends to ruin the whole day not just part of it!