Demon Core
The El Reg side-bar on the Demon Core incident is a fraction too simple.
The Demon core was two hemispheres of Plutonium. The experiements were to bring the two halves almost together to understand criticality.
Unfortunately, the process for ensuring the two hemispheres remained far enough apart was crude: LIke holding a stick or screwdriver between the two halves.
Guess what? The stick slipped. Twice.
The radiation doses killed some of the people in the room.