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. 

'name': name,
                    'quantity': line.qty,
                    'product_id': line.product_id.id,
                    'account_id': income_account,
                    'analytic_account_id': self._prepare_analytic_account(line),
                    'credit': ((amount > 0) and amount) or 0.0,
                    'debit': ((amount < 0) and -amount) or 0.0,
                    'tax_ids': [(6, 0, line.tax_ids_after_fiscal_position.ids)],
                    'partner_id': partner_id
                })
                
                taxes = line.tax_ids_after_fiscal_position.filtered(lambda t: t.company_id.id == current_company.id)
                if not taxes:
                    continue
                for tax in taxes.compute_all(line.price_unit * (100.0 - line.discount) / 100.0, cur, line.qty)['taxes']:
                    insert_data('tax', {
                        'name': _('Tax') + ' ' + tax['name'],
                        'product_id': line.product_id.id,
                        'quantity': line.qty,
                        'account_id': tax['account_id'] or income_account,
                        'credit': ((tax['amount'] > 0) and tax['amount']) or 0.0,
                        'debit': ((tax['amount'] < 0) and -tax['amount']) or 0.0,
                        'tax_line_id': tax['id'],
                        'partner_id': partner_id
                    })
            
            insert_data('counter_part', {
                'name': _("Trade Receivables"),  
                'account_id': order_account,
                'credit': ((order.amount_total < 0) and -order.amount_total) or 0.0,