If you reach the ground within a short period after spawning (using the "Respawn" button after dying), in ~2 seconds or so, you won't receive any fall damage even if that fall would normally kill you. It happens every single time, but if you make the spawn point high enough the reach the ground within ~4 seconds, it doesn't happen. How to replicate:

1. Generate a superflat world
2. Type /spawnpoint @p ~ ~15 ~
3. Type /kill
4. Press "Respawn"
5. Once you touch the ground, you will not take fall damage.