When you fly down with Elytra in a fast speed and you hit a wall the game does not reset the falling speed, resulting in a few hearts of damage taken by hitting the wall (too small and not done on porpouse I think, but I reported this in another ticket) plus 10+ hearts damage for fall damage also if you hitted the wall near the ground

reproducing:
1. Build a pillar 100+ blocks tall
2. Build a wall 10 blocks tall and far from the pillar built before
3. Jump off the pillar, gain a considerable speed and hit the wall very close to the ground, if you done this right you will damaged a bit from the 1st impact and when falled to ground you will probably die

The bug is that in real life you would probably die for impact to wall and, if not, the little fall from wall to ground will not hurt you so much because you stopped falling when hitting wall.

I hope you understood beause I don't know how to explain this better