X-ray using stair/slab and trapdoor

Using a stair, slab, or similar, and a trapdoor, you can see through blocks (as if you were in spectator mode).

To replicate:

1. Place a slab or upside-down stair so that you can move under it if you crouch
2. Place a trapdoor on the ground under the stair/slab
3. Open the trapdoor, crouch, and walk underneath the stair/slab, then close the trapdoor
4. Close the world and relog (you do NOT need to reload the entire game, just the world)
5. You should be able to see through blocks in some directions