Just got a bug reproduced twice just now. After the boreworms dig underground, if you occupy the tile that they dug, or attack them imetiately as they re-emerge to the surface they turn invisible (or fight from underground). They aggro you, they attack and you can fight them back. You can even kill them and the corpse it seems that it remains invisible somewhere - you cannot pick them up, inspect or do anything to them. They remain somewhere invisible or underground, the only presence is that your character gets stuck into the tile occupied by the corpse and has to go around due to the push-out mechanic.
I have 2 such invisible boreworms in my mine right now ... Another mention is that I fought them while mounted on a horse ...
Cape ?
