So I have been working on a cycles advanced material setup that I generate with python. THANK YOU so much for fixing all the python bugs for me, but I've now noticed an issue with node groups. If you open the attached file and change the view in the lower left to render blender crashed (it does this in CPU and GPU mode. I tried to simplify this file down, but it seems if I delete a number of nodes the problem fixes itself. In my material I have a mix by height node group. If I ungroup this node group the problem fixes itself as well. The problem started happening when I added the mix RGB and the RGB to value node into the bottom input of the mix by height node group I would normally research this a ton more and give you more information on exactly what makes it occur, but I'm so busy these days I was unable to boil it down further.