Impacted versions: 
9.0x

Steps to reproduce:
1. Go to Accounting, create customer invoice and validate. Check the Invoice-Number (for example: INV/2016/0010)
2.  Go to Point of Sale and select New Session
3. Add any product or just close & confirm the session
4. Close the POS Session (Validate Closing & Post Entries)
5. Create new customer invoice and validate it. Check the Invoice-Number. 

Current behavior:
Gap between the first and the last invoice because of the closed POS-Session. 

Expected behavior:
No Gap between the two invoices like in Version 8. 

I've tested this behaviour on a 8 and a 9 runbot and my own system. There is no gap on 8 but on 9. I think this is not a feature but a bug. I can't find the missing sequence-numbers in any Journals or in the invoice-list. 

The only way to avoid this, is to select another Sales Journal in the POS-Configuration. But then you can't create an invoice from the POS. 

Is it possible to get the "lost" invoice-numbers somewhere? We are not allowed to have gaps in the invoice-numbering here in Germany. 

'origin': order.name,
                    'partner_id': address.get('delivery', False),
                    'date_done': order.date_order,
                    'picking_type_id': picking_type.id,
                    'company_id': order.company_id.id,
                    'move_type': 'direct',
                    'note': order.note or "",
                    'location_id': location_id if pos_qty else destination_id,
                    'location_dest_id': destination_id if pos_qty else location_id,
                })
                message = _("This transfer has been created from the point of sale session: <a href=# data-oe-model=pos.order data-oe-id=%d>%s</a>") % (order.id, order.name)
                picking_id.message_post(body=message)
                order.write({'picking_id': picking_id.id})
            for line in order.lines.filtered(lambda l: l.product_id.type in ['product', 'consu']):
                Move += Move.create({
                    'name': line.name,
                    'product_uom': line.product_id.uom_id.id,
                    'picking_id': picking_id and picking_id.id or False,
                    'picking_type_id': picking_type.id,
                    'product_id': line.product_id.id,
                    'product_uom_qty': abs(line.qty),
                    'state': 'draft',
                    'location_id': location_id if line.qty >= 0 else destination_id,
                    'location_dest_id': destination_id if line.qty >= 0 else location_id,
                })
            if picking_id:
                picking_id.action_confirm()
                picking_id.force_assign()
                order.set_pack_operation_lot()
                picking_id.action_done()