The bug

When blocking projectiles (could not get it to work with melee), it seems that the player is moved upward very slightly. Normally this is nearly unnoticeable, however when sneaking in small spaces, this can cause the player to briefly collide with the block and take suffocation damage.

How to reproduce

1. Give yourself a shield
2. Create a small space that you can sneak into (bottom slab with a roof 2 blocks above it) 
3. Spawn a mob that can shoot at you (like a skeleton)
4. Block with the shield and sneak far enough into the small space that your head will collide with the block
   →  If done correctly, the player will take a tick of suffocation damage