Piano wire is held at a certain pitch by a pin. In some cases, the tuning pins can go loose on a piano. The block of wood beneath the 200+ tuning pins can crack, usually in a line across multiple pins, making them untunable. The repair is very extensive, requiring removing the action, strings, all the pins, the plate, etc. And costs thousands of dollars. Without this repair, the tuning simply will not last more than a day, or sometimes, minutes.