Moderator Note
The skin system has changed for 1.8 including new layers, new skin templates are required.
Use
[media] as template.
See http://minecraft.gamepedia.com/Skin#Creating_a_skin
I'm not sure of the pattern here, but whenever I have a jacket texture and no sleeve textures, my left arm turns completely black. Will continue updating in the comments as I break it more.
Note that it appears that my (classic) arm texture is turning black, not my (new) sleeve.
Linked issues
is duplicated by 9
Comments 4
Bah, I'm tired and Minecraft is doing different things when it goes into 1.8-compat mode. My mistake.
For the record, old skins should still work.
I was having this issue since I updated my own skin.
It seems having a white background instead of a transparent background, will render the outer layer invisible.
You left out your left arm and left leg in your skin, see the template at
http://minecraft.gamepedia.com/Skin#Creating_a_skin