EDIT: Please read the very last post and Sereja's post if necessary.
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Video: http://youtu.be/fKmmaSlLSvA
The issue is some verts are transforming without them being selected. I'll select some to move, and transform them like I want, but some other completely random ones move as well. If I select just those verts that moved, the other ones move back..
It is something that will hinder me from making models permanently with XSI, it MUST be fixed. Does anyone know a solution? I didn't see any documentation on it..
Try copy your model, and move at some distance in any direction. Then, merge old and new copy in single model (or make a two new copy and merge them), click merge materials, and delete generator inputs. In polygon mode, crop no needed model copy part, and delete it. Now, you should have the same model, but mostly free of bugs. The issue of this method: you can't edit UV's anymore, by selecting some separated parent parts, so you have to edit them first, if it's needed.
This appears to have no effect on the issue.. I still have the same result.
-- I have found the "problem."
There is an option called "Symmetry Mode" in the right side panel with the transform tools.
Make sure that button isn't active and the issue will stop. Evidently XSI can detect symmetric geometry and will utilize it for your advantage when this button/mode is activated. So it is in fact not a bug, but rather a user friendly option. -- Solved.