When you make a stair of Magma Blocks and jump (while holding shift the whole time) you take damage.

Steps to reproduce:

1. Build a stair of Magma Blocks (as can be seen in the screenshots)
2. Change game mode to either Survival, or Adventure
3. Shift-walk (sneak) to the very edge of the Magma Block and jump while still holding the shift key

You will fall down one block (down to the next Magma Block) and you will get damage 1 or 1/2 heart (2 HP)

Make sure to hold "Shift" in all of these steps.