There are a few things that you should never plug into an extension cord. First, any appliance that produces heat, like a space heater or hair dryer, should not be plugged into an extension cord. Second, any appliance with a motor, like a power saw or drill, should not be plugged into an extension cord. nnThese appliances can overload the cord and cause it to overheat, posing a fire hazard. Finally, multiple appliances should not be plugged into the same extension cord. This can also cause the cord to overheat and start a fire. So if you're ever in doubt, it's better to err on the side of caution and not plug it in.