I worked for a large government department and our team were the first to develop PC based solutions, the department's vast majority of work being done on ICL mainframes. We bought our own development server (Novel Netware), wired up our own network, and got ourselves a UPS to protect the server. We used to test the UPS every 3 months, doing a controlled shutdown, and everything was hunky-dory.
About 18 months later we moved to a new building and our server was moved into a new server room and plugged into the new network. We got it up and running very quickly and things were going well. Until, that is, we decided to run our regular UPS test. We switched off the mains supply, then watched as the software on the server detected the power out and shutdown everything gracefully. At this point screams came from another team that their system had gone down.
When putting their server into the server room, that had spotted a spare outlet on our UPS and decided that it would be a good idea to hook into it, but they had not made us aware of this. Doh!