When holding your shield inside of certain damaging blocks, the shield blocks the damage.

How to reproduce:

1. Give yourself a shield.
2. Create a fire or place a lava bucket/magma block/cactus/sweet berry bush/campfire.
3. Switch to Survival mode.
4. While blocking with the shield, walk into the block you placed.

Expected result:

Shields would not be able to block environmental damage sources.

Observed result:

Shields can block damage from standing in fire, in lava, on magma blocks, in sweet berry bushes, on campfires, next to cactus, or in the area a lightning bolt strikes.