A lot of Chaos Knight's cosmetic items are missing backfaces. Most of them are from the sets.

Note: I'm limiting this now even further. This is a list of all items which have visible holes in them in the loadout menu and the default camera angle. I leave out items which have no backfaces on normally not-visible spots.

The following items are missing backfaces

1. His default set: Missing backfaces on the horse's chest armor and cloths, on Chaos Knight's leg armor as well as below the shoulders.
2. Arms of Burning Turmoil set: Missing backfaces on the chains of the horse and its armor all around (chest, head, legs, hooves and back), and below the shoulder pieces.
3. Chaos Legion set: Missing backfaces the horse's nose armor and on the two metal pieces hanging below its head, and on Chaos Knight's shoulder pieces and the entire cape.
4. Charge of the Baleful Reign set: Missing backfaces on the horse's neck armor, behind CK's helmet and below CK's shoulders.
5. Embers of Endless Havoc set: Missing backfaces below the chest plate of the horse and the cloths hanging down its sides, and below CK's shoulders.
6. Rising Chaos set: Missing backfaces on the inside of CK's weapon, below his left shoulder, and on the entire bone armor of the horse.
7. The Hounds of Chaos set: Missing backfaces below the shoulder pads.
8. Catastrophe: Missing backfaces on the cloths hanging on both sides of its neck.
9. Atramentos the Apocalyptic: Missing backfaces on its leash.
10. Chaos Knight's Armlet of Mordiggian: Completely missing backfaces, and not even adapted to CK's arm size. His arm could fit like 3 times in that armlet. The item is very hideous in the loadout menu, because it's 80% transparent there.