When attempting to load a world with FastCraft 1.21 and COFHTweaks 3.0.3-303 installed,
the game shows a single frame of a black screen and then exits.
Log: http://pastebin.com/0TGtwiP1
(Confirmed with only FastCraft, COFHCore 3.0.3-303 and COFHTweaks, happens consistently every time.)