My personal peeve is thumbscrews on cable connectors. Guys who design connectors should be forced to spend 45 continuous minutes personally fastening and unfastening them. If they still have all the skin on their knuckles and no blisters on their fingers, it's a pass.
As for cars, when I was in the service a friend had a big old Chevrolet of some variety (no longer recall) with a V8 engine. By today's standards, vast amounts of room in the engine compartment, but still some bone-head layout decisions. I got drafted to remove two of the eight plugs whenever he did a tuneup because I'm shorter than him and my elbow was in the right place to swing the sparkplug wrench!