Pontiac was not supposed to build the 1964 GTO at all. A corporate rule banned engines over 330 cubic inches from the compact Tempest, so three engineers found a loophole in the option book and dropped a 389 in anyway. Management approved 5,000 units as a hedge against failure. Buyers ordered more than six times that number in the first year, and Detroit spent the next decade chasing what Pontiac started by accident.