The mistaken belief that a rocket will be stable if the motor is ABOVE the rocket's center of mass, so that the rocket is "hanging" below the center of thrust and will try to remain upright, like a hanging pendulum. It doesn't work; even Robert Goddard learned that the hard way. See the following links for more information: http://en.wikipedia.org/wiki/Pendulum_Rocket_Fallacy and http://www.geocities.com/jim_bowery/pendrock.html