The bug: Holding F3 + A and not letting go gives you the ability to clearly see nametags and particles through blocks like from players or lava particles for example.

Steps to reproduce:

1. Move players/mobs with nametags and lava where you can't see them
2. Hold F3 + A to x-ray through blocks to see players/mobs and lava (make sure you aren't too far away). If you computer freezes try lowering the render distance.

Expected behavior: You shouldn't be able to see anything through blocks, not even nametags so you can't see them easier due to the contrast from the sky/void.

Actual behavior: You are able to see particles through walls and nametags a lot easier as if there aren't any blocks in between.