An early example of this can be seen on the Atari VCS (2600) console. With the "Space Invaders" game, one could hold down reset when powering on the machine and get a two shot mode. As word of that got around, people would rapidly cycle the power, and or partially insert the power cord into the receptacle on the rear of the machine to get odd game effects and states.
Was this ever done to any useful effect on vintage / retro computers?