Had a problem at a school a couple of years ago. We had an aprentice go to all the recetacles with the plug-in tester. All rec's showed wiring OK. Most of it was in block work with a 4" sq. box above drop ceiling for termination. There was 120 for power and 277 for lighting. One of the lighting whips was mistakenly tied into the receptacle j.b.. Tester read circuit OK. This tester doesn't tell you voltage. Later on a carpenter plugged in his drill and without even touching the trigger it jumped, spun, smoked and flew out of his hand. I don't know if anything like this ever happened to anyone, but it was a bad mistake, specially in a school.
