An employee and co-worker arrived at a customer site to work on a six-burner range oven. Upon arrival, they found the gas lines were flooded with water and the oven was waterlogged. They turned off the gas to hook up a safety valve and thermostat to the main gas line. They turned the gas back on and started to do a leak check with leak detection solution. They sprayed fittings, lines, and connections on valves. There was no indication of leaks. They turned the gas on and went to light pilots to test the gas. Upon lighting the pilots, there was a flash that came out of the front valve and the back of the oven. The injured employee was standing in front of the oven and suffered burns to their arms, hands, and face.