I was helping a friend with a new plane he had built. It passed all the usual checks and I was about to take off when it stopped responding to signal input. I pulled the fuel line to kill the engine and we pulled the wing to check things over. The fuel tank had developed a leak so bad the bottom of the servo bay was awash in glow fuel. He had positioned the battery connection lying flat on the bottom of the bay. The fuel had entered the connection and opened the circuit. If this had happened a few minutes later, it would have been one of those crashes that you're not sure what happened. It would have been impossible to determine if the leak had occurred before or as a result of the crash. I'm going to check all my planes to make sure there are no connections located on the bottom of the bays. You never know when a tank might split.