Leaving the door open on a microwave oven does not result in a significant amount of energy loss. Modern microwave ovens are designed with safety mechanisms that prevent them from operating when the door is open. These safety features typically include interlock switches that disable the microwave's power source when the door is not properly closed.
While a small amount of energy may escape through the gaps around the door, it is generally negligible and won't have a significant impact on energy consumption or efficiency. However, it's still a good practice to keep the microwave door closed when it's not in use to maintain proper functionality and prevent any accidental exposure to microwave radiation.