You can reproduce this without the assistance of plugins:

1. Throw snowballs at a living entity (e.g. Pig) from afar (>3 blocks away). The living entity will get hit by the snowballs without any problems.
2. Gradually walk closer to the entity.
3. At close range, the snowballs begin to hit the player who is throwing them, not the living entity!

Note: this was not a problem prior to 1.9. I believe the "setShooter" function of projectiles used to have a grace period that prevents the projectile from hitting the shooter when it is first launched.

This bug reduces the effectiveness of all plugins that rely on projectile-based combat.