Star Trek: 10 Classified Starfleet Secrets
2. The Secret Of Talos IV
The true nature of Talos IV was a secret so guarded by Starfleet that revealing it was, during The Original Series, the only crime with a death penalty.
Talos IV was the planet visited by Captain Pike and the old Enterprise crew in the Original Series pilot episode The Cage. The world was home to the Talosians, whose technological illusions were so powerful that they could allow extremely injured or dying people to live comfortably again. Spock commandeered the Enterprise in the two-parter episode The Menagerie in order to free his former Captain, Pike, from his wheelchair.
The law forbidding travel to Talos IV was known as General Order 7 and, as mentioned earlier, had a punishment of death, but Spock was able to protect his crew by seizing control of the ship single-handedly, risking only his own life. In the end, Starfleet made an exception to General Order 7 and allowed Spock to go unpunished, but the law likely remained in place afterwards at least for a few decades.
Starfleet feared what would happen to both the Federation and the Talosians if the public were to learn of the incredible powers found on Talos IV.