The hardest aid routes are poorly protected, requiring the climber to make long sequences of moves using hooks or tenuous placements. On these routes, a climber may have to commit to moving up onto the most marginal of placements risking long and sometimes dangerous falls. By contrast, the vast majority of aid ascents are done on popular free climbs which are too difficult for the aid party to free, but offer excellent gear placements. Since aid climbing is extremely slow compared to free climbing, this can lead to some conflicts between aid climbers and free climbers waiting to climb a route. There is additional tension caused by the damage that aid climbing often does to routes. Hooks frequently break or otherwise damage holds that human hands and feet do not. New aid climbers also often compulsively "bounce test" pieces the reliability of which experienced leaders can often assess at a glance; removing a "bounce-tested" nut often requires hammer blows which further expand and sometimes fracture holds.