I've tried dragging various number of entries left, right, up, and down, individually and in snakelike combo, with varying degrees of difference.
I've tried with one level 2 nesting, and I've tried with several.
I've tried level 3 nesting.
I've tried starting with the mouse cursor on the top item, the bottom item, and several stages in between.
I can't imagine why some of those might matter, but I tried them anyway.
I CANNOT get this error to show up.