It looks to me like Visio doesn't like it when I delete a member of a group. I think it sometimes leaves a little remnant behind like a ghost or a skeleton. Every once in awhile, after I have been grouping and ungrouping shapes, I will see an outline of a shape when I ungroup. If I lasso that area, I will see the known members of the former group plus a phantom shape (rectangle). If I clear the selection, I cannot seem to select that one element. But if I unselect each of the other group elements, it will be left and I can delete it.
I haven't been able to replicate it yet. If I do, I'll post more information.
These ghosts do not seem to cause any problems other than to confuse me.
Is this a known problem?