When you make a tower of sand/gravel and start breaking it form the bottom, a shadow will stay there. It can be fixed by putting the blocks again and break it from top to bottom or reloading the world. Possibly affects other platforms, just not tested.
Steps to reproduce: (image name in brackets ())
1-Find a bright area (Bright area).
2-Make a sand/gravel tower (Make a tower)
3-Break the tower from the bottom (Break from bottom)
4-The shadow will be there (Shadow)