This occurs very rarely and is most likely to happen if you knock your computer sharply. During normal operation, the disks inside the hard disk drive are spun by a motor. The magnetic head that is used to read and write data onto the disk's surface actually has two tiny wings and flies just above the surface of the disk. If you knock the PC, the magnetic head could wobble and touch the surface of the disk. If this happens, it'll damage the surface of the disk and you could lose some data. In a very bad case, you could wreck the disk drive.