While riding a horse you can spot a weird grid like pattern on the wall. The horse must be near a wall to replicate this, I've already reported this to B4nj077 (Fredrik) over Twitter and have recieved no response but i believe he has access to the JIRA.

HOW TO REPRODUCE

> Spawn a horse in an enclosed area
> Tame the horse and place a saddle and diamond gear on it
*> Ride the hourse while accessing F5 mode and you will notice as the horse glitches through the wall a small black pattern emerges.