One of the few common light configurations on a three-light tester is one that is labeled as a “Hot-Ground Reverse.” This is a very rare situation that would be very difficult (and dangerous) to achieve. Assuming that the ground wire is connected all the way back to the panel, it would require the ground bus in the panel to be energized to 120 volts and the black wire (normally hot) to be connected to ground. So, yes, this is indeed a rare occurrence in actuality. However, it is not too difficult to achieve a totally different wiring configuration that can cause a 3-light tester to give the indication of a hot-ground reverse.
The situation that can give this indication starts with a disconnected neutral upstream of the receptacle being tested. The only other thing needed to get the 3-light tester to show a hot-ground reverse is to have a load plugged into the receptacle in question or into a receptacle downstream of this receptacle, and for that load to be turned on. The load that is plugged in will not be working because the disconnected neutral upstream prevents the current from flowing, but the plugged-in load will cause the 3-light tester to indicate a hot-ground reverse because the plugged -in and turned -on load will essentially interconnect the hot and neutral slots of the receptacle, causing them to both be at 120 volts.
Here is why this happens. Since the disconnected neutral prevents current from flowing, the 120-volts that enters the load will go through the load and out the neutral side of the plug, causing the neutral side of the plug to be energized also to 120 volts. (Remember, when there is no flow of electricity, there is no voltage drop across the load.)
Let’s look at an example of a lamp being plugged into the receptacle. When there is a proper neutral, and the current is flowing, the voltage drops from 120 volts on one side of the light bulb to 0 volts on the other side. (See diagram below.)
Now, let’s look at the situation when the neutral wire is disconnected somewhere upstream of the receptacle shown. When the neutral wire back to the panel is not connected, creating an open circuit, everything in the circuit stays energized at 120 volts. This is because with no current flowing, there is no voltage drop. This means that everything in the open circuit is at 120 volts. This also means that both the hot and neutral wires that are connected to the receptacle are at 120 volts. (See diagram below.)
In the circumstance above, a 3-light tester will measure 120 volts between the hot and the ground, causing the right-hand light (on most testers) to come on. It will also measure 120 volts between the neutral and the ground, causing the left-hand light to come on. Since both the hot and neutrals are at 120 volts, it will measure zero voltage between the hot and neutral wires, causing the center light to stay off. Thus, we have the left-hand and right-hand lights illuminated which is interpreted by the 3-light tester as a “Hot-Ground Reverse.”
This is just another example of how you can be led astray by one of these 3-light testers. If you are a home inspector, you can at least report that there is a problem with the receptacle, although you may be wrong about what the problem is. If you are an electrician trying to diagnose why a particular receptacle is not working, you can definitely be misled about what the problem is if you rely solely on one of these 3-light testers. In either case, the more you know about the equipment that you are using, the more useful that equipment can be for you.