This is the gist of this annoying lil' bug:

I thought that the textures in my resource pack would load correctly.

Instead, they either lost a few steps, started to misplace certain pixels, and it would keep the broken textures. I even redid them 5 times and it still failed.

Steps to Reproduce:

1. Download and use the texture pack so it's easier to understand
2. Place the 16 colors of wool
3. Find the textures in the resource pack and fix them (easier to do with texture pack, right?)
4. Save the file
5. F3 + T in-game
6. Feel utter disappointment and triggered.
7. Give up because this is gonna keep on happening.