Usually happens if i go to the Over-world from another dimension. Last time I saw it was on CubeCraft Games, Minerware, Large rectangles of shadows that look dark and lighting smoothness is non-existent. When you go into it, your skin and, if you're holding one, your map in you're off-hand darkens and player arrows darken the further into the shadow you go.

Unfortunately I wasn't able to get a screenshot.