I noticed that when you press F11 to go from windowed to fullscreen, the game redraws everything, but it redraws entities (like mobs, chests, spawners, and pots) first before redrawing blocks.  This lets you for a split second see these entities through the walls in caves, directing you to more caves and loot.  It seems to do this repeatedly if you keep pressing F11.

It's easier to see during daytime, as the night background makes it harder to see when looking through the world.  It also won't always show nearby mobs, as sometimes they are culled by the game if they haven't been close to being rendered yet.

Expected behavior: You should not be able to see through solid walls.