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
string="Starting Balance",
        help="Total of opening cash control lines.",
        readonly=True)
    cash_register_total_entry_encoding = fields.Monetary(
        related='cash_register_id.total_entry_encoding',
        string='Total Cash Transaction',
        readonly=True,
        help="Total of all paid sale orders")
    cash_register_balance_end = fields.Monetary(
        related='cash_register_id.balance_end',
        digits=0,
        string="Theoretical Closing Balance",
        help="Sum of opening balance and transactions.",
        readonly=True)
    cash_register_difference = fields.Monetary(
        related='cash_register_id.difference',
        string='Difference',
        help="Difference between the theoretical closing balance and the real closing balance.",
        readonly=True)
    journal_ids = fields.Many2many(
        'account.journal',
        related='config_id.journal_ids',
        readonly=True,
        string='Available Payment Methods')
    order_ids = fields.One2many('pos.order', 'session_id',  string='Orders')
    statement_ids = fields.One2many('account.bank.statement', 'pos_session_id', string='Bank Statement', readonly=True)
    picking_count = fields.Integer(compute='_compute_picking_count')
    _sql_constraints = [('uniq_name', 'unique(name)', _("The name of this POS Session must be unique !"))]
    @api.multi
    def _compute_picking_count(self):