Cleaning is one thing... though it can also be damage to the flat green cable going from the keyboard to the motherboard. When the "lock" on the white socket at the motherboard is pressed down and someone tries to just jank out the green cable you'll get scraches on the green cables exposed contact area. This can probably be repaired with cunductive silverpaint but as the exposed contactarea is normaly larger than needed I usually just cut off a few mm and that've worked so far. :-D
