Let's see.

• While the script was running, I tried File - Close from the Poser menu, closing the scene, which did create an error.  This is not really a good idea anyway while a script is running.  PPy scripts are generally linked to the currently open scene and closing that scene would seem to me to be a surefire way to crash most scripts.  I don't know if that's what you're reporting, however.

• I'm not sure about File/Source.  That may be a Poser option peculiar to OS X?  Were you trying to display the script's source code?  I think Windows does that with ctrl-click on the script menu button....  (Just tested that, and no crash here.  I suspect I'm testing the wrong things....)

• You won't see any changes to the vertices of the box in the preview window, because the script alters texvertices and doesn't do anything with vertex positions.

• The script doesn't currently alter the UVs of the selected actor.  It could be set up to do that, perhaps.  Right now, it creates a new copy, with the altered UVs, of the selected actor.  To make a copy, press the Make Copy button.  The new actor will be automatically selected by the script when created, so you can test the results by closing the script and opening it again.

Umm.  Does any of that help?  Ultimately, this is intended to work with some specific situations for TDMT, but I'm trying to integrate some broader utility....

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.