Be cautious with old, non-polarized extension cords (with both prongs the same size). If the cord is plugged in the opposite way that it should be, several types of appliances may become a shock hazard and will have high electric field. If an appliance has a polarized plug, use only a polarized extension cord (with one small and one large prong).
The outer case of any plugged-in appliance is supposed to be connected to ground, but sometimes it becomes accidentally connected to the electrical “hot” if the polarized plug is inserted upside down. Because of the possibility of this improper grounding, you should never touch a plugged-in appliance while taking a bath, or while touching the metal of a sink or the water stream in a sink. Also, plug-in appliances should not be allowed to get wet, because water makes it more likely that the outer case will become at least weakly connected to electrical “hot”.