I hired a furnace company to repair a gasket in a hot water tank, which they did for $500. A few days later it was leaking again. They came back and did the same repair without following standard trouble-shooting methodology, which would call for them to try to determine why the first repair failed immediately. The 2nd repair failed immediately. I thought about it for a bit, and decided that excessive water pressure could cause the problem. Checked with the water company. Sure enough, pressure was very high in my area. Got a plumber in, found the pressure regulator valve had failed.