Power issues caused the May 16, 2014 downtime, as stated in Stack Exchange Primary Data Center Offline - May 16th, 2014
What happens when you plug a UPS into itself? , a post for a picture contest, talks about it being caused by a UPS being plugged into itself.
I know that the photo in the post itself is a joke. But was the failure being caused by a UPS being plugged into itself also a joke, or real?
Yes, plugging a UPS into itself sounds like a pretty stupid mistake to make, but I assume it's possible when you've got a large and complex system.