Falling Sand on a Slimeblock doesn't bounce, but makes a grafical glitch and turns directly into sand, gravel, anvil, enderegg and more falling blocks I might forgot about.
How to reproduce:
1. Place a slimeblock
2. make a tower on 1 side
3. Place sand on the side where the slimeblock is, on top of the tower
4. See the bug
(Gamemodes don't effect this)

In this youtube video I show you the bug aswell:
http://www.youtube.com/watch?v=pvZRoE35G48

Since this bug is resolved for being a suggestion:
This is not a suggestion but really a bug, all entities should bounce, they do bounce, the sand does bounce a little, but it becomes solid in less then 1 gametick