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. 

prec_acc = self.env['decimal.precision'].precision_get('Account')
        pos_session = self.env['pos.session'].browse(pos_order['pos_session_id'])
        if pos_session.state == 'closing_control' or pos_session.state == 'closed':
            pos_order['pos_session_id'] = self._get_valid_session(pos_order).id
        order = self.create(self._order_fields(pos_order))
        journal_ids = set()
        for payments in pos_order['statement_ids']:
            if not float_is_zero(payments[2]['amount'], precision_digits=prec_acc):
                order.add_payment(self._payment_fields(payments[2]))
            journal_ids.add(payments[2]['journal_id'])
        if pos_session.sequence_number <= pos_order['sequence_number']:
            pos_session.write({'sequence_number': pos_order['sequence_number'] + 1})
            pos_session.refresh()
        if not float_is_zero(pos_order['amount_return'], prec_acc):
            cash_journal_id = pos_session.cash_journal_id.id