The bug

When you throw a projectile like a snowball or egg, the 2D sprite is offset upwards from the real position of the projectile, no matter what direction you're facing.

How to reproduce

Run:

/summon snowball ~ ~ ~ {NoGravity:1}

and look straight downwards. You would expect your crosshairs to be pointing at the center of the snowball, but the bug makes the snowball look like it's slightly higher.