When falling onto slime blocks wearing an Elytra, your momentum does not go down.

Steps to reproduce:
1. Make a slime block platform
2. Wear an Elytra
3. Make sure you are looking directly down
4. Teleport yourself up a few blocks (/tp @p ~ ~5 ~)

You won't ever stop bouncing. If you open F3 you'll notice you'll always be bounced up to the same coordinate.