This will be just a quick post to explain something that I have recently discovered. The low cost, 3-light outlet testers that many home inspectors and electricians use can give you different readings depending on whether or not something is plugged into the outlet or into an outlet downstream.
I was recently doing a home inspection and came across a receptacle in the garage. The washing machine was plugged into this outlet when I plugged my tester into it. The lights on my tester lit up indicating a Hot/Neutral reverse on the outlet. I decided to do a quick experiment by unplugging the washing machine. As soon as I unplugged it, the indication on the 3-light tester changed to showing an open ground on the outlet.
I have since built a small test rig to check other configurations and the indications that the 3-light outlet testers give under given situations. With this test rig, I have verified that something that is plugged into an outlet downstream of the outlet being tested will give the same result as I saw above at the home inspection. This means that if you are a home inspector or an electrician and all you are using the check the electrical outlets is one of the 3-light testers, then there is a very good chance that you are getting and reporting some incorrect readings.
The linked video shows this phenomenon very clearly.