We have had two instances lately where phantom Invoice numbers get assigned to closed SOs. We just figured out this morning that if you go in to view a closed SO and somehow change the RTS field to Y, that the SO reopens and issues a new Invoice number.
 
Our settings are - Allow Reopen SO? == V, view only. Also, shipper number == Invoice number
 
Because the SO has no lines it will not show as Shipped, Not Invoiced or available to invoice. It is just a missing Invoice number, which I then have to find by going in to BKARINV.B with Maintain Database.