Broken: 2.79 5bd8ac9
Short description of error
Copying objects by Ctrl+C with custom material nodes (created in python by class MyCustomSocket(bpy.types.NodeSocket) .... ) produced undefined behavior in Blender, after which Blender crashes.
Raw diff how to fix this bug
Did more deep investigation of this bug inside Blender's source code and found how this could be fixed. Here it is raw diff: https://pastebin.com/SzwwpCnn