A safety test at half past one

Reactor 4 at Chernobyl was destroyed during a test of what would happen if the power failed, run at night by a shift that had not been briefed on it.

The design had a flaw at the tips of the control rods that briefly increased reactivity when they were inserted, so the shutdown button made it worse. Sweden noticed the fallout two days later on a routine check of workers' shoes, which is how the rest of the world found out.