As mentioned elsewhere:

 

- open 2 grids, side by side

- drag/drop a subitem from 1st grid to the other grid, under another item (parent)

- a new parent link is created BUT the old link is preserved even if ctrl isn't pressed. (I.e. : moving the item creates 2 parents links, instead of just changing the link.)

 

This could obviously result in many unwanted links.