Gears and pinions can be damaged in many ways. The most common would be wearing of the teeth. A gear in a clock train wears the teeth in one direction of rotation. On some clocks, a gear can be removed and turned over to run on the other side of the teeth of the gear. In most cases, it is wiser to have the gear remade.
Other damage to a gear or pinion can be caused by a mainspring breaking. The power released when a mainspring breaks can break teeth out of a gear. Although gear teeth can be repaired, the better solution is to have a new gear made.