The Legion of Super-Heroes #259

The Legion battles the Psycho-Warrior, a man driven mad by an accident in space that gave him fantastic mental powers. The villain shows Superboy how his parents will die. He won't remember the knowledge of his parents' death when he goes back to the 20th Century but would remember that terrible moment every time he returns to the 30th. To spare him that, Saturn Girl plants a hypnotic command so that Superboy will never again join the Legion.