Friend of mine in an A320 was minding his own business in the cockpit and he had his head-set on. A ground-handler walked to the external jack and plugged in his own head-set. An intensely loud piercing electronic squeal went directly through the captain's ear-phones as the ground handler plugged in and the captain's hearing was so badly damaged that he lost his licence for several years. Hand signals do have their advantage at times like these...