Watched a guy chase a ghost for 6 hours on a King Air in Tulsa, still bugs me
He swore the #2 nav was drifting but it turned out a 3 inch section of the antenna coax had chafed through on a clamp behind the panel, and I only found it because I ran my hand along the whole run instead of trusting the tester.
A tester would've had me chasing my tail too, but honestly running your hand along the whole run is just plain common sense. Six hours of swapping boxes when it was a chafed coax behind the panel the whole time, that's a rough one lol.