Cable caps
The cable caps for the rear brake are truly essential, although owing to paint thickness, it may initially be hard to get them in the frame opeinings. But if you don't install them, the act of braking will strip the cable and pull it into the frame. There has even been one report on the site of this happening even with a cable cap installed. I haven't checked the diameter of derailleur cable caps and brake cable caps, but it could be that it is important to use the correct type in this instance.