I was trying to add a combined Copper/Coal site.
I selected the Copper then I selected the Coal. It added a Coal only site.
I added the Copper site on it's own. Then I added to the Copper site a small patch of Copper that got separated.
Last I tried adding to the Coal site the smaller half of the Coal that was separated and the crash was instant.
Sorry for the screenshot, didn't feel like typing all that in
