Per Moesh's comment on MC-46956, I'm making a new report for this.

When mobs are on teams they can be hit by projectiles from teammates, unlike players.

Steps to reproduce:

1. /team add TestTeam
2. /team modify TestTeam friendlyFire false
3. run a few times:

   /summon skeleton ~ ~ ~ {Team:TestTeam,HandItems:[{id:bow,Count:1}]}

4. get the skeletons to attack you
5. notice how some of the skeletons take damage from other skeletons' arrows.

Expected result:
Friendly fire is respected and the mobs be hit by teammates' projectiles.

Affects all forms of non-melee damage, so arrows, fireballs, dragon breath, splash potions, evocation fangs, etc.