Switching off the oxygen
Some years ago, a past workplace of mine got one of those systems installed which could deoxygenate the server room in the event of a fire. The control panel was mounted right by the door, and unfortunately was very sensitive to jostling, like the vibrations from the door slamming because someone was carrying something bulky in and didn't have a hand free to close the door gently.
Luckily, this system would wait a few seconds before actually activating, giving someone the chance to stop it first. So those of us non-admins who worked nearby remained blissfully unaware of the hazard until the inevitable day when someone didn't manage to deactivate it in time. A lazy summer afternoon was interrupted by the almighty FFFFWRRRMMM of the system activating, followed by a couple panicky minutes of realizing what the sound must have been, trying to locate someone with server room access who was not actually in the server room at the time, visualizing colleagues passed out from hypoxia... (It was not until after this incident that we were told it didn't lower the oxygen level so far as to become totally unbreatheable.)