If you are a home inspector or an electrician and are using one of the low-cost, three-light outlet testers to check electrical outlets, you could be missing a wiring problem that you should be aware of. These three-light outlet testers will NOT indicate a hot/neutral reverse situation if the outlet being tested is not grounded.
If you are testing an ungrounded outlet, then all the tester can tell you is if there is a voltage differential between the narrow slot and the wide slot on the outlet. Without also having a ground wire connected, the tester does not know if the wide slot or the narrow slot is the one that is hot because it does not have a point of reference. Remember, with alternating current, the hot wire is at negative 120 volts as much as it is at positive 120 volts, so the tester has no way to know which of the two wires is actually the hot wire. It simply knows that there is a 120-volt difference between the two slots.