The texture for the flames is missing.
DO THIS (to recreate it):
I played Minecraft in single player and was in the Nether. A blaze put me on fire with one of his fireballs.
Current behavior (14w25a):
While burning I can only see (from a first person view) the default violet/black square for a missing texture across the lower half of the screen.
Expected behavior:
There shall be flames!
TODO:
IMHO this should be checked.
Already reported in MC-57778, my bad, sorry!