So... I was changing some of my insulated piping to normal piping while the game was paused, I don't think I was doing much out of the ordinary, just flicking between views. And then the game crashed and this popped up:
The game music kept playing in the background but I had to wrangle with the Task Manager to actually stop the game to be able to re-run it.
As a side-note: it's possible to replace tiles from material A into material B (regular tiles), it's possible to replace piping from insulated -> normal and the other way around, but not insulated piping from one material to the other? The reason I was turning my Insulated Ingeous Rock piping into regular one was to be able to rebuild it as Insulated Ceramic piping.
The save file's here:
Steps to Reproduce
Replacing some insulated piping with regular one. The game crashed.
This issue has not been confirmed by a developer yet.