I had one pick out a faulty keyless fixture. First AFCI I ever installed, put it on some K&T and it tripped immediately. Began isolating parts of the circuit until I traced it back to an attic keyless. Wouldn't trip on a new keyless, but went every time on the old one. No signs or sounds of arcing, so I'm guessing it was the GFP function. Don't have a megger but a multimeter didn't find any low resistant connections.

Who knows? [Linked Image]

-John