Affects Version/s: Minecraft 1.9 Pre-Release 4, Minecraft 1.9
Fix Version/s: None
You can become invulnerable to all knockback by following these steps:
1) Create a superflat creative world in Minecraft 1.9 Pre-Release 4
2) Run this: /tp @a -415 4 1465
3) Run this: /tp @a -798 4 1231
4) Go into survival mode and summon in all the monsters you want - you no longer take knockback until you die or reload the world.
Don't ask me how it works, those teleport commands magically make the player invincible to knockback.