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_session.py
self.ensure_one()
        self.write({
            'login_number': self.login_number + 1,
        })
    @api.multi
    def action_pos_session_open(self):
        
        
        for session in self.filtered(lambda session: session.state == 'opening_control'):
            values = {}
            if not session.start_at:
                values['start_at'] = fields.Datetime.now()
            values['state'] = 'opened'
            session.write(values)
            session.statement_ids.button_open()
        return True
    @api.multi
    def action_pos_session_closing_control(self):
        for session in self:
            for statement in session.statement_ids:
                if (statement != session.cash_register_id) and (statement.balance_end != statement.balance_end_real):
                    statement.write({'balance_end_real': statement.balance_end})
            session.write({'state': 'closing_control', 'stop_at': fields.Datetime.now()})
            if not session.config_id.cash_control:
                session.action_pos_session_close()
    @api.multi
    def action_pos_session_close(self):
        
        for session in self:
            company_id = session.config_id.company_id.id