So, i'm using PolyMc as my mod pack handler.
So far, I have tried 1.6.9a-G and 1.6.8-Gv5
Whenever I start a world everything goes completely black and breaks the main menu and world.
I haven't tried the same modpack in a different modloader, it may be a issue with polymc but i don't know.

Ok, i managed to fix it, all you have to do is go to the config and disable enableShaders2