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
ctx = dict(self.env.context, force_company=company_id, company_id=company_id)
            for st in session.statement_ids:
                if abs(st.difference) > st.journal_id.amount_authorized_diff:
                    
                    if not self.env['ir.model.access'].check_groups("point_of_sale.group_pos_manager"):
                        raise UserError(_("Your ending balance is too different from the theoretical cash closing (%.2f), the maximum allowed is: %.2f. You can contact your manager to force it.") % (st.difference, st.journal_id.amount_authorized_diff))
                if (st.journal_id.type not in ['bank', 'cash']):
                    raise UserError(_("The type of the journal for your payment method should be bank or cash "))
                st.with_context(ctx).sudo().button_confirm_bank()
        self.with_context(ctx)._confirm_orders()
        self.write({'state': 'closed'})
        return {
            'type': 'ir.actions.client',
            'name': 'Point of Sale Menu',
            'tag': 'reload',
            'params': {'menu_id': self.env.ref('point_of_sale.menu_point_root').id},
        }
    @api.multi
    def open_frontend_cb(self):
        if not self.ids:
            return {}
        for session in self.filtered(lambda s: s.user_id.id != self.env.uid):
            raise UserError(_("You cannot use the session of another users. This session is owned by %s. "
                              "Please first close this one to use this point of sale.") % session.user_id.name)
        return {
            'type': 'ir.actions.act_url',
            'target': 'self',
            'url':   '/pos/web/',
        }