Is there any danger? Contrary to what many people think, reverse polarity can also be a serious safety concern. Consider a light fixture with no bulb. If wired correctly, the only live part is the button at the base of the socket. A person is much less likely to touch this button than the threaded collar around the socket. If wired with reversed polarity, it is the threaded collar that is live! If you inadvertently touch the metal base of the bulb while it was in contact with the socket I can receive a severe shock.
Switched Appliances - Polarity matters with appliances that have switches. When the appliance is plugged in, power should only go as far as the switch. If polarity is reversed, power will go through the entire appliance back to the switch. If a wire comes loose in the appliance, the entire case of the appliance may be electrified, even though the appliance is not on. This is a shock hazard. Reversed polarity outlets often go unnoticed for a long time. Many appliances will work just fine.
