The Bug
Glass, when in item form, can be used to see through transparent blocks. That includes chests, water, ice, piston heads and glass itself. Some of the more recently added mobs like horses, illagers and all aquatic mobs are also affected by this

Steps to reproduce
-Place a semi transparent block in the floor
-Put glass in your hand
-Go into F5 mode (it'll also work with dropped glass)
-Look towards the block, making sure the glass is in the way