Same thing happens with cars. Dashboard says everything is fine, then your alternator dies on the highway at 11pm. Or a home inspector runs his little tester, says the wiring is good, but the outlets still flicker when the AC kicks on. The machine only checks what it was told to check. It doesn't know about the worn out part or the loose ground or the guy who did a half ass install 15 years ago. Test gear is a tool, not a truth teller. The old timers know that because they got burned enough times to stop believing the screen.