The Bug

Mobs can be flung out of the world border in the nether if hit with a weapon that has knockback (any level, as long as it can fling the mob outside the border)

How to reproduce

1. Go to the world border (or set the world border to a place very close to where you are)
2. Give yourself a weapon with Knockback.
3. Find or spawn a mob right at the edge of the world border.
4. Hit the mob with your weapon. The mob should be flung outside of the world border.