Shimano BioPace? Except that gets re-invented about once a decade or so...
Early bonded alloy/carbon frames and forks. Alan's come to mind. A few others like that are just plain hazardous to ride.
Tioga Tension Disk.
Shimano U-Brake
Onza clipless pedals were fiddley, but controversial for some.
There was a company that did some plastic discs that clipped onto your spoke wheels for a more aero profile. Those were a pain.
Slightly off topic, the one I miss the most are the little wire tire sweepers. No room in the average frame for them any more. Very controversial to show up at a local competitive ride with those things back in the day.
They used to sell a tear-off kit for those Oakleys. Worked for me. Dusty NORBA racing was far better with the tear-offs. Not enough people racing for them to be relevant now.