An energy anomaly that interfered with a transporter beam and returned four of the Enterprise-D's company, Picard among them, to the bodies of twelve-year-olds in the episode Rascals.
The molecular reversion field is an energy anomaly met by the shuttlecraft Fermi in The Next Generation episode Rascals in 2369. As the shuttle breaks up, its four passengers, Picard, Guinan, Ro Laren and Keiko O'Brien, are beamed out through the field.
They materialise as children of about twelve, with adult minds and memories intact. Beverly Crusher finds that the field caused the transporter to drop certain genetic sequences that govern maturity, and that the four would simply grow up again in the normal way. Picard, unable to command in that form, hands the ship to Riker.
When renegade Ferengi in surplus Klingon Birds-of-Prey seize the Enterprise, the four use their harmless appearance to move freely and retake the ship with help from Alexander Rozhenko. O'Brien and Crusher then restore them by passing them through the transporter again using their stored adult patterns.