This one actually happened to another tech in my company, but it is worth sharing. He was doing a PM on a card access system at the world headquarters for a re-insurance company. The doors leading into the server room had magnetic locks on them, which require an over-ride on the inside of the door, typically a pull station or break glass device (break the glass which releases a button held closed), so he tested the break glass right next to the door, or so he thought, too bad it was the EPO for the ENTIRE UPS system (which was not identified), he said all you heard were computers spinning down and most of the lights went out. At the end of the day the customer actually thanked him because they always saw the red button on the wall but didn't know what it was, still not something I would want to be responsible for.
