After a couple in-game days in a world, a few rectangular shadows with no object casting it will appear in some places. It isnt even a complete chunk that's affected, either. Just a few random rectangles that are less than a chunk.

No, this is not a real shadow. It is not casted by anything, and reloading the world removes it (just to reappear after a while).
The shadows seem to disappear along the player's z-axis, as well (as shown by the two pictures that depict a line right at my feet
If a flower spawns in the shadow, the transparent parts of the flower's texture will become visible.
When it occurs in water, the surface texture of the water becomes invisible
In addition, sometimes, blocks inside the shadow will glow.
Finally, REAL shadows cease to exist along that aformentioned player's axis.