Oh, what a bummer! Sorry about that. I have seen that happen once before; my wife sent me a screenshot of one of her games. What did the symmetric room in the upper left of the fortress look like, I wonder? I’ll look through the code and see if I can think of what went wrong.
Thanks for the bug report! I built some diagnostics and finally located the bug, which only happened (occasionally) in fortresses with biaxial symmetry. I'm testing the fix and should get it released soon.