The destruction of the Earth vessel SS Valiant in 2065 by order of its own captain, after an encounter with the energy barrier at the galaxy's edge gave a crewman dangerous psychic powers.
The SS Valiant was one of Earth's first deep-space ships. A magnetic storm swept it out to the rim of the galaxy, where it tried to pass through the energy barrier. Several crew died and one survivor began to develop extrasensory abilities at a frightening rate.
The ship's tapes show the captain searching urgently for records on psychic phenomena and then giving the order to destroy his own vessel.
Two centuries later, in the Original Series episode Where No Man Has Gone Before, the USS Enterprise recovers the Valiant's ejected recorder marker. The warning comes too late to prevent the same change overtaking Gary Mitchell and Elizabeth Dehner.