In the nether, I saw a naturally-spawned baby zombie pigman roaming around. However, when it got near to a wall of netherack I heard it take damage. Observing it for longer, I saw that it was taking damage whenever it entered a space in the wall that was one block high. It wasn't suffocating, but it always took damage (I'm unsure how much) when walking/jumping from the block below into the one-block-high space in the wall. It was as if it was taking damage for entering that space, but not for being in it.

I was unable to test this in creative because I can't spawn baby zombie pigmen. I also do not know how to recreate this glitch. Screenshot attached of the baby zombie pigman I mentioned just after I met it.