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Details follow - Spoiler Alert (only read if you are beyond chapter 1)
So I am doing something I haven't done for years - playing through the OC. Just to spice it up a little, I am playing as a cleric. I am currently only up to the point where I am looking for the waterdavian creatures - specifically for the Yuan Ti. So I am in the warrens of the damned, using turn undead a lot and destroying most of the zombies. Up to this point the Yuan Ti has been speaking through selected zombies and then destroying them. Up to the last 2 of these special zombies I had no problem. However, after traversing the north/south corridor to the west I decided to continue around and clear the east corridor heading south. In the corner area with the big hole to the southwest my PC was attacked by a zombie, so I executed a turn undead. Unfortunately this apparently interrupted one of the special zombies from conveying whatever the Yuan Ti was saying. This meant that an unkillable (must have the plot flag set) zombie became hostile. Fortunately they move slowly so I continued south. The same thing happened a bit further down the corridor with the result that there were 2 unkillable zombies after my PC. The only thing that got rid of them was actually killing the Yuan Ti.
So is this a bug or was I just unfortunate?