Bug occured again, this time with a twist.
What happened: I played a game, died to a giant spider, and started a new skirmish game. This time, the game started my sovereign in the middle of the ocean, and turned him into the spider that killed him.
I have to say, I'm really impressed with the fortitude and determination of these guys. Even being eaten by spiders isn't enough to stop them from bringing life back to the dead world: they're gonna do it from inside the belly of a giant arachnid if they have to!
I guess that's why they're the Sovereigns.

And here's the Debug.err file:
http://pastebin.com/m27ced0
On a smaller note, on the game where I got eaten by the giant spider, I tried building a mine on a patch of ore, but no image popped up: the mine built and everything, just didn't show the mine on the screen.