An employee had been working on a piece of equipment from the basket of an aerial lift. The employee tried to retract the boom to back away from the equipment. The boom unexpectedly began to lower, and the basket contacted the handrail of a catwalk that ran alongside the equipment. The lift continued to lower. The handrail bent and two of the lift's wheels began to leave the ground. The lift moved away from the equipment; the basket came free from the handrail and returned to the ground quickly and with enough force to knock the employee off his feet. He suffered injuries to the shoulder and ankle (both requiring surgery), as well as lacerations to the forehead and one cheek.