In the AM I clecoed the trailing edge skins in place. Headed for the airport but got a call on the way cancelling the lesson. Went back to work and began clecoing and riveting the whole assembly together. Finished the trailing edge skins and was working on the leading edge when the pneumatic puller flew apart. The back part had apparently worked loose and came off. The metal part became an unguided missile, grazed my shoulder, and ricocheted off the wall. Lucky it didn't hit me in the face. Called Taylor Tools, and they advised I send it in for repair and recalibration. Glad to do that, as the tool never really has worked right. However, now I am without the puller and the flaperons are not done. I may be able to finish the left one using the hand puller, but no way am I doing the whole right one by hand. Will have to adjust the order of my next tasks to adjust to the delay.