Reverse polarity -- fixed.
Using this handy testing tool I found eight outlets with reversed polarity this weekend. Fixing it was fairly straightforward -- cut the power and rewire the outlets so the hot and and neutral wires are connected properly.
If you live in an older house I'd highly recommend getting one of these since reversed polarity can harm sensitive electronics and creates a shock hazard.
Don't forget the non-contact voltage tester so you don't fry yourself while fixing the old outlets. Some old homes didn't even have a ground wire run, so DIY'ers may have just hooked the ground to the neutral in the j-box rather than rewire the entire house. The tester won't catch that.
And don't forget to test again after turning the power back on :-)
Also helps find bad GFCI outlets. Completely essential.
This is a great tool. Most receptacles today are labeled or use two different metals for each side to make it easier but like you said, if you live in an older house, those receptacles you just hooked the wires up and you were good, they didn't worry about polarity.
James Feragola, Sep 06 2011 on 1500wordmtu.com