Reported by Joe Rose:
When in "vertices editing mode" on a line/polygon shape, and working quickly, one can easily shift the whole shape when trying to add a vertices (double click on segment) and (not by desire) moves the mouse simultaneously. This occurs because the edit/add vertices operation is in the same logic "plain" as the move shape operation. and if wishing to add a vertices and then reposition it one can very easily move the whole shape. When this occurs numerous times in a vertices adding/editing session it cause a shape "drift" which cannot be fixed without restarting the fixes/changes from the beginning. When working with a shape of 100,000 of more vertices and having changed a few thousand the consequence of an "drift" error is, well, NUTS.
I would suggest that the moving shapes be kept if on another logic (shifted) plain than the move vertex option. The shifted one should be the version least used by the users as this requires the extra key press. Hard on us old guys with arthritis when doing hundreds or thousands of operations (as I do at times).