It would seem like that wouldn't it :) The problem is this.
The items are NOT deleted. When we use the dropdown from within the work order to select them - it displays as it should in the page.
When we then create an invoice from that work order, it moves the number over and all the notes (number being item number) and puts it into the invoice.
In order to "fix" the fact that the error is being thrown, we must recreate that full line item by creating a new line, selecing the SAME ITEM EXACTLY from the dropdown and then updating the grid and deleting the original.
We know it is the same number (item number) because we named the original as (old) and created these NEW numbers, and those are the ones that we select.
It happens with only 3 items.