When your minions rebel and start attacking your stuff, defences and other minions will fight back against them. However, slow-moving projectiles (bombard shots, crackpot thrown potions, bafu spines, etc. - basically, any travelling projectile) will go right through the rebel without dealing damage. Ranged attacks that do not use a projectile, such as the Witch Doctor's 'lightning from above' attack, work fine.
I believe that projectiles like this rely on hit detection, and are coded not to detect hits with friendly units. This check doesn't seem to be taking into account the difference between actually-friendly units and rebels.
To reproduce, I used the Hell's Deep pet dungeon map with god mode enabled. You can easily create a rebel by placing any reasonably tough creature and slapping them repeatedly.
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