Abstract: Employee #1 and a coworker were blocking the inside walls of a townhouse. Another coworker was working from a wood bench, using a Duo-Fast nail gun, serial #D916F, model #IN124, to toe the inside walls of the roof truss framing. Employee #1 was leaning through a window to lift two bulkheads into place with the help of his coworker, who was at the other end of the bulkhead. The second coworker, who was toeing the wall near the window through which Employee #1 was leaning, dropped the nail gun to his side with his finger still depressing the trigger. The nail gun struck Employee #1 in the back with enough force to depress the work contacting element and discharge a nail. The nail penetrated the employee's left upper back at an approximately 45 degree angle, creating a puncture wound approximately 4 1/2 in. below the top of his shoulder and 4 in. from the left outside of his body. Employee #1 did not require hospitalization. The coworker using the nail gun had been with the employer approximately one month and had been using a nail gun for two to three weeks. He had, however, used a nail gun at other jobs for other employers, and he had been given on-the-job training by a supervisor and a blocker. Both considered him to be competent in the safe operation of a nail gun. His employer at the time of the accident did not have specific safety rules or written procedures for the operation of nail guns, but employees were told not to keep the trigger depressed. Immediately after the accident, the nail gun was removed from service. It was subsequently inspected by maintenance and found to be in proper working order.