Oil The micro-drops of oil that lubricate the pivots will dry over time and depending on how the watch is used. Unoiled pivots will seize the mechanism, causing the watch to run slow, fast or to stop. They can also become scratched and/or break.
Wheels The many wheels in the watch's movement are constantly moving and will eventually show signs of wear. A tooth might break or the wheels rub one against the other. This friction can cause resistance which reduces the watch's autonomy (power reserve) and accuracy. If a watch has been stopped for a prolonged period, the wheels can seize up. For this reason, a watch should be made to function at least 2 or 3 times a year.
Mainspring The mainspring on a hand-wound mechanical watch can break if roughly wound. The mainspring on any watch will, over time, lose some of its elasticity resulting in reduced autonomy for the watch.
Balance and spring The balance and spring are in constant demand, making several thousand vibrations per hour. Any rough or sudden movement will subject this complex construction to shock, leading to errors of rate.