When traveling through the overworld, I have encountered several instances of an area that appears to be in the shade, despite no opaque object above it to create a shadow. The first instance was a single block beneath a tree, the leaves cast a shadow far darker than the other leaf blocks. This resolved when the leaf block above was broken. The most astounding was in a mountain biome, in an area with nothing above it at all. This was far larger, measuring 63X14 blocks. Mobs spawned in this area despite full sun above them, and my player character became completely darkened when it entered the area.