Minor spoilers ahead if you care about scenario rooms:
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On lichenous Lair levels there's some sort of meteorite scneario room wich also shows a different 'burnt' grass tile.
Lichens seem to get stuck or something because they dont follow me outside of that burnt grass. They freeze there and dont nothing. They hardly fight back too.

Pushing them with weapons or shield works and they return to normal that way.

http://imageshack.us/photo/my-images/593/spiral20120208185502.png/
http://imageshack.us/photo/my-images/69/spiral20120208163617.png/