Abstract: Employee #1, a piecework carpenter, was nailing down roof sheathing when the pneumatic nailing gun he was using apparently ran out of #8 nails. Employee #1 reached up the roof to get more nails from a box but he lost his balance and started to slide down the roof. To stop himself from sliding, Employee #1 reached out with his hand and accidentally hit the nail gun nozzle (with operational pressure-retracting safety device) against his knee and depressed the safety and the trigger. The remaining nail shot into the employee's knee. Employee #1 admits that he was trained on the proper way to handle the gun by his foreman, but stated that he had his finger on the trigger. The gun was tested and could not be made to fire without the nozzle safety and the trigger being depressed. No apparent tool malfunction was involved. The accident was caused by employee error or inattention.