You have a group with lines and polysurfaces. The lines are a group in the group. When you _isolate the lines; switch on the editpoints; select some editpoints with ctrl+shift and move them - sometimes I find after _show the polysurface in the group has moved too.
I was not able the repeat the problem - hence the ominous question.
Hi Frank - I think this might be very easy to do by accident, actually, since you may not see the difference, with the polysurface hidden, between Ctrl-Shift selecting top level objects within the group and normal selection which will include the hidden objects - when dragging, the hidden object is shown but it may not be in view.