You can goober things up if you merge Runtimes. There is at least one file inside the Runtime that will kill other versions of Poser. I have a copy of the factory default runtimes for P4, PP, P5, and P6 so I can check things out in different versions of Poser. I've accidentally copied the wrong runtime into a version of Poser and Poser wouldn't work until I deleted that runtime and put the correct one it. There's at least one .dll that causes issues because each version of Poser wants a different version of that .dll. (There could be other files as well, but I saw no need to start experimenting on all the ways I could kill Poser.) Cheers!