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_order.py
if self.product_id:
            if not self.order_id.pricelist_id:
                raise UserError(
                    _('You have to select a pricelist in the sale form !\n'
                      'Please set one before choosing a product.'))
            price = self.order_id.pricelist_id.get_product_price(
                self.product_id, self.qty or 1.0, self.order_id.partner_id)
            self._onchange_qty()
            self.price_unit = price
            self.tax_ids = self.product_id.taxes_id
    @api.onchange('qty', 'discount', 'price_unit', 'tax_ids')
    def _onchange_qty(self):
        if self.product_id:
            if not self.order_id.pricelist_id:
                raise UserError(_('You have to select a pricelist in the sale form !'))
            price = self.price_unit * (1 - (self.discount or 0.0) / 100.0)
            self.price_subtotal = self.price_subtotal_incl = price * self.qty
            if (self.product_id.taxes_id):
                taxes = self.product_id.taxes_id.compute_all(price, self.order_id.pricelist_id.currency_id, self.qty, product=self.product_id, partner=False)
                self.price_subtotal = taxes['total_excluded']
                self.price_subtotal_incl = taxes['total_included']
    @api.multi
    def _get_tax_ids_after_fiscal_position(self):
        for line in self:
            line.tax_ids_after_fiscal_position = line.order_id.fiscal_position_id.map_tax(line.tax_ids, line.product_id, line.order_id.partner_id)
class PosOrderLineLot(models.Model):