Level 15 light-emitting blocks (sea lanterns in my case) cast shadows.

How to Reproduce:

Take a sea lantern and place it about 15 or 16 blocks in the sky. Directly under the lantern will be a shadow. There can be a shadow even at night.

Realistically, lights brighter than the moon (light level 4) should not cast shadows on the moonlit ground. Less luminous blocks such as glowing obsidian should have similarly lighter shadows, while jack o' lanterns, beacons, and campfires do have a clearly defined non-luminous base which should cast a shadow.