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. 

addons/point_of_sale/models/pos_order.py
if not self.test_paid():
            raise UserError(_("Order is not paid."))
        self.write({'state': 'paid'})
        return self.create_picking()
    @api.multi
    def action_pos_order_invoice(self):
        Invoice = self.env['account.invoice']
        for order in self:
            
            local_context = dict(self.env.context, force_company=order.company_id.id, company_id=order.company_id.id)
            if order.invoice_id:
                Invoice += order.invoice_id
                continue
            if not order.partner_id:
                raise UserError(_('Please provide a partner for the sale.'))
            invoice = Invoice.new(order._prepare_invoice())
            invoice._onchange_partner_id()
            invoice.fiscal_position_id = order.fiscal_position_id
            inv = invoice._convert_to_write({name: invoice[name] for name in invoice._cache})
            new_invoice = Invoice.with_context(local_context).sudo().create(inv)
            message = _("This invoice 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)