The GFCI Line/Load Wiring Mistake
A simple plug-in circuit tester, costing less than a fast-food meal, can identify a line/load wiring mistake in seconds.
Why this mistake is so easy to make and so hard to notice
The terminals are typically labeled directly on the device, but in a crowded electrical box it's easy to land the wrong wire on the wrong screw. Because the GFCI itself often still passes power and even trips/resets normally when tested, the mistake can go unnoticed indefinitely — the failure only shows up in what it isn't protecting downstream.
How to check for it
A plug-in circuit tester at both the GFCI outlet and any outlets believed to be downstream of it can reveal whether protection is actually reaching those locations. If you're not confident interpreting the results, this is a quick, inexpensive check for a licensed electrician to confirm during any electrical work.