The Fainsa Sophia. Promised to be a good seat, shaped similarly to and with similar firmness to the Grammer E3000 Desiro seat, but some idiot decided it should have a base frame with a step in it* and a thin cushion which means you can feel the base through it after about 20 minutes. Awful thing that needn't have been so bad if the base frame had been a flat metal plate.
Furthermore, TOCs (Thameslink, ScotRail, LNR 730s) who think this abomination of a seat is suitable for First Class! The ScotRail staff when I tried the 385 mockup were not impressed when I told them Standard was considerably more comfortable.
* I think the *idea* is that your weight goes into the deeper part and you need less cushion under your upper thigh, but unfortunately that's designing it for only one shape of person. It might even have worked if the step wasn't as, er, stepped.
The fad of pointlessly tiny luggage racks in early-2000s designs like Voyagers, *Stars and 175/180s.
Lack of gangways on 195s when Northern intended to multiple-work them.
Seats by blanks and luggage racks by windows on Pendolinos. Really wouldn't have been hard to, you know, swap them over?
Seats by 80x door pockets. Just put a luggage rack there!
Turbostar internal doors. Did they ever work?