This is not the same issue as what it is linked to as being a duplicate, this was not in Peaceful mode. It has to do with the player's legs being inside the entity of a lit TNT in Normal difficulty only dealing 1/2 heart of damage instead of causing the player's death. I tested it without changing any settings and having myself next to the TNT instead, it did 10 hearts and killed me.

What I expected to happen was...:
Instant death (Upon it's detonation) due to being inside the entity of a piece of lit TNT.
(no armor, survival mode, inside the entity of a lit piece of TNT)

What actually happened was...:
I took 1/2 heart of damage and walked off just fine.

Steps to Reproduce:
1. Dig a 1x1 hole that is 3 blocks deep.
2. Place a single piece of TNT inside.
3. Light the TNT. (flint&steel is fine)
4. Jump down into the hole so you are standing inside the lit TNT.
5. Wait for it to detonate.
6. Find it only deals 1/2 heart of damage instead of the expected 10+ hearts.

It may affect more versions, I was messing around in the snapshot and noticed it. Other versions are untested.