Earth Leakage #2
It passed the Earth Resistance test, the Insulation Resistance test but failed the Earth Leakage test.
Subsequent investigation found that in certain rotational positions of the chuck, it became live and thus failed. The chuck was found to be live over a rotation of 5° out of 360°, making it very unlikely to have been discovered with a standard Insulation Resistance test.
Anybody using it would have been inadvertently playing Russian roulette.