If it's helpful at all, here is more detail on what I was doing immediately before the crash. I was placing the carpenter's slopes on the Botania area that I'm working on. I had completed an inverted pyramid and went to work on the next one. On this next one, as I placed the slopes they were not behaving. No matter what I did, they were placing themselves as some kind of roof-looking block (as though four rooflines were coming down together to a point?). I placed and broke maybe a dozen trying to get them to behave. Then, in my laziness to possibly avoid having to go and get the carpenter's hammer from the AE system, I right-clicked with the wrench and <poof>.