"...all Boeing 787 Dreamliners have to be switched off every 248 days. If they are not reset then the generator control units GCUs will go into failsafe mode and the plane will lose all electrical power.
To quote the FAA directive:
This condition is caused by a software counter internal to the GCUs that will overflow after 248 days of continuous power. We are issuing this AD to prevent loss of all AC electrical power, which could result in loss of control of the airplane..."