Let me give you one example. 120V branch circuit feeding a standard portable lamp in your home. The plug on the lamp is polarized IE; has one prong larger than the other so it can only go in the receptacle one way. If the polarity on the receptacle is reversed the screw shell of the lamp will be at 120V potential when the lamp is turned off. Very dangerous.
If you're asking about neutral and hot conductors being reversed consider: Hard wired garbage disposal, neutral is switched, leaving continuous hot to G/D at all times. Plumber goes to change it out. He shuts off switch and thinks all's well. As he removes wiring from under sink, he encounters hot wire while laying across the grounded plumbing=dead plumber.
Re: Reversed Polarity#6701 01/10/0207:39 AM01/10/0207:39 AM
The screw shell on a lampholder is required to be connected to the grounded circuit conductor only. You are much more likely to contact this portion of the fixture while changing a bulb. If it is an outdoor porch light and you are barefoot... goodbye. Also, as in the GD example above, an inadvertant ground fault inside the motor may turn the unit on without the switch being closed.
Re: Reversed Polarity#6702 01/12/0209:36 AM01/12/0209:36 AM
Thanks all. I am/was referring to reversed polarity at the receptacles. If I am understanding most of you than the main concern is that the item that is plugged into the receptacle may become energized on the conductive exterior of the unit. Is this correct? As for the Plumber that is working on the D/W, I agree but shame on him for not shutting off the breaker prior to working on the unit.