So, me and a couple of friends were just hanging in teamspeak, all minding our own business, when one guy said he had invincible trash mobs on Act 3 Inferno.
Of course we didn't believe him at first, but we were soon convinced otherwise.
We all joined and had our try and the only one doing damage to them was apparently the monk (but very little). Nonetheless we only managed to drop them to 1hp each at which point they became invincible.
I remember seeing a video about a week ago in which someone would get large-hitbox mobs stuck on a staircase, and farm the mobs behind them that couldn't get past with complete impunity. Then, when this invincible-mob bug happened to me recently, I figured it was a stealth nerf to prevent killing something that was stuck on pathing, since that's the only time it's happened to me.