Liquid oxygen
During a routine cryogenic oxygen tank stir in transit to the Moon, a fire started inside an oxygen tank. The most probable cause determined by NASA was damaged electrical insulation on wiring that created a spark that started the fire. Liquid oxygen rapidly turned into a high-pressure gas, which burst the tank and caused the leak of a second oxygen tank. In just over two hours, all on-board oxygen was lost, disabling the hydrogen fuel cells that provided electrical power to the Command/Service Module "Odyssey". This required an immediate abort of the Moon landing mission; the sole objective now was to safely return the crew to Earth.