I was playing sivir and threw my boomerang blade at pantheon as he put up his E towards me.
As expected, he was immune to the damage as the projectile went towards him, however he was also immune to the damage when the projectile hit him from behind on its way back.
Same with GP Barrels. It is not where the Damage Source is, but where the inflicting Person is.
It blocks all damage if he faces the person hurting him. If a gnar throws his boomerang, it wont hurt him if he is facing gnar, and it wont block its return damage if hes facing the other way.
Not a bug that's how it works.
If he is facing you then you do no damage period, even if you were to play 
and use your ult.
That's terribly concerning. I recently discovered that Orianna's ult has a "turn them to face the ball" effect right before the yank. That could lead to some... interesting interactions between Ori R and Pantheon E.
