Inadequate Joint Face Fusion

The fusion of the edges must be clean and complete, otherwise the weld won't hold. If you don't properly melt the edge, you'll get a cold spot that can act like a crack and the weld will break. Make sure the torch really catches the edge and doesn't just push the weld pool ahead. Set the energy so that the edge melts along, and check the weld root on both sides. If in doubt, better do a test run on scrap before you go to the workpiece.