So I was in a plot-relevant battle and things were going... badly. Every time I stepped anywhere NEAR the main badguy, I instantly took large amounts of damage. Enough that if I stood there for more than a moment I would die. Yet nothing I did seemed to scratch him in the slightest.
Finally I backed off... and realised he was now hemmed in by NPCs who were technically on my side (that is, they were blue and attacking him) but not COMPLETELY on my side (not green, not controllable). And he was apparently not scripted to attack them. He couldn't move to follow me, so I could back off and get a breather.
And time passed. And time passed. Round after round of endless whiffing on the part of all the NPCs, and zero response on the part of the baddie. It became clear that nothing could harm the baddie except a critical hit. Every now and then, one of the NPCs would roll a 20 and therefore actually be able to do a point or two of damage. This... wasn't really going to achieve much any time soon.
I was slightly curious if something weird would happen if they actually managed to kill him, but it was FAIRLY obvious that the game had gone wrong somewhere and I didn't really have the patience to sit for an hour while a supposed-to-be-unkillable boss was popped by pins, so I reloaded until I could get his teleport-away trigger to fire properly.