According to Osho, from the thirteenth gunasthana no one has ever fallen, nor is falling possible. The natural current carries one into the fourteenth—final liberation—unless, out of supreme compassion, one consciously chooses to remain embodied to guide others (the bodhisattva/Tirthankara resolve). This pause at thirteen is exceedingly arduous, but it has occurred in rare, strong beings whose presence becomes a lamp for the world.
You can’t slip backward from the 13th stage; you either move into full freedom or, if very compassionate, you stay briefly to help others.