Here's what happened. Mike left a skeleton in the garrison, and Tazar rushed off on a fly. After the turn ended, Tazar had an army of 1 fly and 0 skeletons, and 1560 movement points. The game crashed immediately when hovering over Tazar. No autosaves helped. I had to do a replay.
In general, there are many amusing bugs related to duplications. The other day, Neo told about a situation - a hero dies from skeletons, and then when trying to kill the skeletons, a battle starts with that hero. The fact is that after the hero's death, his "shadow" remained, and another hero "stepped" on it when attacking the skeletons. I don't know if he saw this in SoD or ToEE.
There were also cases where "the hero died and his shadow completely blocked the path."
It can be fixed by rolling back the save to the moment of duplication or by editing the save.
The majority of serious and unfixable bugs in Full and Light stem from bugs in SoD. Like various duplications, save corruption, etc. In fact, Full and Light didn't add much, and what they did add is usually easy to fix or work around.
Added 9 minutes later
In that game, the number of players was probably simply off by 1. Lyokha received the Bone Dragon rank :D