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. 

AND absl.id IN %s 
                GROUP BY aj.name
            """, (tuple(st_line_ids),))
            payments = self.env.cr.dictfetchall()
        else:
            payments = []
        return {
            'total_paid': user_currency.round(total),
            'payments': payments,
            'company_name': self.env.user.company_id.name,
            'taxes': taxes.values(),
            'products': [{
                'product_id': product.id,
                'product_name': product.name[:20],
                'code': product.default_code,
                'quantity': qty,
                'price_unit': price_unit,
                'discount': discount,
                'uom': product.uom_id.name
            } for (product, price_unit, discount), qty in products_sold.items()]
        }
    @api.multi
    def render_html(self, docids, data=None):
        company = request.env.user.company_id
        date_start = self.env.context.get('date_start', False)
        date_stop = self.env.context.get('date_stop', False)
        data = dict(data or {}, date_start=date_start, date_stop=date_stop)
        data.update(self.get_sale_details(date_start, date_stop, company))
        return self.env['report'].render('point_of_sale.report_saledetails', data)