http://en.wikipedia.org/wiki/Demon_core
First incident
On August 21, 1945, the plutonium core produced a burst of neutron radiation that led to Harry Daghlian’s death. Daghlian, a physicist, made a mistake while working alone performing neutron reflection experiments on the core. The core was placed within a stack of neutron-reflective tungsten carbide bricks and the addition of each brick moved the assembly closer to criticality. While attempting to stack another brick around the assembly, Daghlian accidentally dropped it onto the core and thereby caused the core to go critical, a self-sustaining prompt critical chain reaction. Despite quick action in moving the brick off the assembly, Daghlian received a fatal dose of radiation. He died 25 days later from acute radiation poisoning.
Second incident
On May 21, 1946, physicist Louis Slotin and seven other Los Alamos personnel were in a Los Alamos laboratory conducting an experiment to verify the exact point at which a subcritical mass (core) of fissile material could be made critical by the positioning of neutron reflectors. The test was known as “tickling the dragon’s tail” for its extreme risk.
...Under Slotin’s unapproved protocol, the only thing preventing this was the blade of a standard flathead screwdriver, manipulated by the scientist’s other hand. Slotin, who was given to bravado, became the local expert, performing the test almost a dozen separate times, often in his trademark bluejeans and cowboy boots, in front of a roomful of observers. Enrico Fermi reportedly told Slotin and others they would be “dead within a year” if they continued performing it.
On the day of the accident, Slotin’s screwdriver slipped outward a fraction of an inch while lowering the top reflector, allowing the reflector to fall into place around the core. Instantly there was a flash of blue light and a wave of heat across Slotin’s skin; the core had become supercritical, releasing an intense burst of neutron radiation estimated to have lasted about a half second. He quickly flipped the top shell to the floor. The heating of the core and shells stopped the criticality within seconds of its initiation, but Slotin’s reaction prevented a recurrence and ended the accident. Slotin’s body’s positioning over the apparatus also shielded the others from much of the neutron radiation. He received a lethal dose of 1000 rads neutron/114 rads gamma in under a second and died nine days later from acute radiation poisoning.