EE was tightening the fasteners on the black plastic pipe (48""diameter) for the #1 kiln stack. EE had 7/8"" wrench in left hand and a 1/2"" impact with a 3/4"" socket in right. When they pressed the trigger on the impact, the nut caught a burr on the bolt threads and the wrench spun out of hand and slapped EE across the knuckles.