Along with the lost neutral reason found in the dryer thread, a few years ago I got a call that a elderly home owner was being shocked by a freezer in her garage, my first response was to use my 3-light tester in the receptacle it was plugged into since the freezer was new, well the correct wiring lights lit so I started to do some more testing with an extension cord plugged into a known good receptacle and my wiggy, what I found was the neutral in about 4 receptacles were tied to the grounding terminal, and new NM ran back under the crawlspace to a junction box where they were connected to old cloth covered non grounded NM, the color was no longer distinguishable from hot to neutral and they had tied the neutral to the hot by mistake, so in retrospect, this could have caused a death and cost the life of this little old lady, one more reason I have a lot of respect for the code and doing things the right way.
Remember we can be held liable and even criminally liable for what we do, a good thing to keep on your mind when doing electrical work.