When you stand on fence posts that are 2 blocks high and are in 3rd person mode a black box appears behind the player and follows the player's camera. Note that in the previews you need to be crouching to be able to see this bug. 

Steps to reproduce:
      1) place fence posts to make a 2x2 wall
      2) go into 3rd person and walk into the fence post backwards

      3) if you are in preview, crouch when your player's feet look like they are floating above the ground (standing on the fence posts)

Video of bug