Love these threads..
As for my experience, it is a little limited compared to some commenters, but here goes.
When I was a student, our Unit had a nice, shiny new(ish) building right on the banks of the Thames. The main computer room, along with the inlet for the electrical supply for the building was in the basement. Which, being next to a river, flooded regularly.
We also had a lab in the basement of another building on that campus. That wasn't on the banks of the Thames, but still flooded whenever it rained. Probably not a good idea to put Tower PCs in that lab, and we spent many hours cleaning motherboards and repairing PCs that had been in a flood.
My best one though was I was called in by a friend and ex-colleague to advise his directors on video streaming. The only time available for me to go see what hardware they had was a Saturday, so we went in to his work on a hot Saturday. Unfortunately, their server room was two 8 foot racks packed with servers, and a few assorted PCs in a tiny room with no windows and a small Air Con unit that failed. I have no idea how hot it was, as the thermometer in the room maxed out at 40 deg. C, and the mercury had gone past that to the top of the tube. What I do know is that the bulk of their hardware had overheated and shut down, and the couple of servers that were running had overheat warnings on their LCD displays. I also know that we were sweating like crazy almost the instant we went in there, and the only option available was get every fan in the building, take it to the server room, and try and get some cooler air in there. After two hours, the room was merely uncomfortably hot (as was the rest of the building), but it was cool enough we were able to get the servers restarted. On the plus side, that two hours did give me a chance to go into the local O'Neills with my friend and his then girlfriend, have a Full English and essentially get paid for it.
On the plus side for him, he suddenly got all the funding he needed to install the monitoring and alert system he'd requested a while before and was denied.
Where I am now, I'm not too involved in the server side of things (although I do get involved if needed), and the few times we'd had electrical problems, the UPSs have enabled the servers to shut down gracefully. Having said that, one time it happened, National Power had been drilling in the road outside and put a drill through our mains. That was fun for me. I was in the lift at the time, with four women. Something which isn't as fun as it sounds as one was slightly claustrophobic and I spent my whole time trying to stop her mild anxiety turning into a full blown panic attack. There was also another time when another colleague was working in our main server room, and one of the phases blew.. Apparently, he has never heard such a loud explosion. After his hearing returned, he (and his team) spent the next 3 days determining what machines we needed, testing them and plugging them into the other circuits.