The bug

While on fire, the player is often immune to fall damage. From my testing, this does not seem to affect other kinds of damage-over-time effects like poison, or other types of damage like enemy attacks.

How to reproduce

1. Get on a high pillar and place fire
2. Enter survival mode, get lit on fire, then jump off
3. Land
It's not perfectly consistent, but is relatively easy to recreate

VIDEO:
firebug.mp4