A vault containing a single active nuclear warhead in pristine condition, and a red button that activates it, in case of emergency. Of course, the red button is covered by a bulletproof glass housing that only a key possessed by the Overseer will unlock. Of course, the lock on the glass housing is broken, and can be opened and activated by anyone.
In addition, the air ventilation system has a serious problem, in that a faulty fan blade unreachable by vault citizens intermittently lets off an ear-piercing shriek that, due to the echoing nature of the filtration system, ricochets around the vault, usually emitting from a ceiling grate just over the shoulder of a random vault resident. This happens on average around 100 times a day, making sure a tenth of the vault's 1000 residents experience it in a given day, assuming no overlap.
This, however, would prove to be a fallacious assumption, as for whatever reason certain individual citizens hear it much, much more often than others, to the point that many vault citizens question whether the noise exists at all.