I've noticed in the latest snapshot that I cannot draw back the bow if I am targeting any mob when less than 5 blocks away from the player.  If I move to 5 blocks away I can draw the bow back.  This affects targeting for any passive or hostile mob.  If I draw the bow back when not looking at a mob within 4 blocks, I can release the arrow without problems, but not if I am looking at the mob when doing so.