I once got this which did not make sense to me at first:
"Parent item has a child item that is in a different list: You can only add a child to a parent item, if any existing children of that parent item belong to the same list."
We had a subset as a parent of a numbered list. We tried to update the subset. Item could not be removed from the subset (parent list) because it had a child item in the numbered list. This is logical, although my opinion is that you should be able to remove the item from the subset and the child item would be left as orphan. This was year ago, haven't tried if it functions still in the same way.