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I entered the thieves' lair with a hero and stayed there until I needed to delete that hero. In the same turn, I brought in another hero to the lair and spent the night there. On the next turn, I pressed the spacebar, and a transfer window appeared with the dismissed hero. I left the lair and hovered over the lair's cell, and the cursor turned into a sword. I moved, and... I attacked a dead creature (the hero was Isra; dead creatures cannot die:smile04:). During the battle, when it was the skeleton's turn, the game crashed.
Loading (thanks to the guys for the battle saves) easily fixed everything. I'm just telling you because it was a funny bug.