It has been fun reading this!
I think the worst farkles are ones I've bought and never gotten around to installing, after having spent a bundle on them.
2 come to mind: the FJR unpainted and undrilled tire hugger that's been lying around for about 10 years, and the Kevlar ABS brakeline kit, which, with 7 or more hoses and a lot of complaints about missing connectors never got installed.
OTOH, some of the problematic farkles on my FZ1 never made it to the FJR, like the Corbin seat which was harder than stone and slippery, and NEVER broke in. Then there was the Day-Long seat. I had to send it back to them 2 or 3 times because it pushed me too far into the FZ1 tank. I finally rebuilt it myself and discovered that inside it used a 3/8" piece of plywood as a support! Now it's OK since I cut a ton of foam out of the back support and changed the angle of the seat foam, but it cost far too much to be worth it and I wasn't happy with their support. I tried a Day-Long on someone's FJR at one of the Port Jervis rides and it had the same problem as mine. It SHOULD be comfortable in theory but isn't.
The throttle paddles didn't bother me but I found they didn't work well enough to be worth it.
But the worst farkle of all was a very, very expensive intercom/phone/music system (was it Starcom?) before Blue Tooth became common. Was it StarCom? Not sure anymore. It wouldn't work with my phone--their answer was "get another phone", the helmet speakers had to press against your ears, painfully, to be able to hear them and get any audio quality. Their answer: "That's the way it's supposed to be. You'll get used to it!" The switching between my CD player and the phone (when I found a phone that worked with it) took minutes, rather than seconds. Finally, the CPU for the thing was heavy, ungainly, and wouldn't fit anywhere practical on the bike. I ended up selling it on eBay at a big loss, just to be rid of it! The Nolan built-in BlueTooth systems are far, far superior!