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. 

'session_id': current_session.id,
                'date_order': fields.Datetime.now(),
                'pos_reference': order.pos_reference,
            })
            PosOrder += clone
        for clone in PosOrder:
            for order_line in clone.lines:
                order_line.write({'qty': -order_line.qty})
        return {
            'name': _('Return Products'),
            'view_type': 'form',
            'view_mode': 'form',
            'res_model': 'pos.order',
            'res_id': PosOrder.ids[0],
            'view_id': False,
            'context': self.env.context,
            'type': 'ir.actions.act_window',
            'target': 'current',
        }
class PosOrderLine(models.Model):
    _name = "pos.order.line"
    _description = "Lines of Point of Sale"
    _rec_name = "product_id"
    def _order_line_fields(self, line):
        if line and 'tax_ids' not in line[2]:
            product = self.env['product.product'].browse(line[2]['product_id'])
            line[2]['tax_ids'] = [(6, 0, [x.id for x in product.taxes_id])]
        return line
    company_id = fields.Many2one('res.company', string='Company', required=True, default=lambda self: self.env.user.company_id)
    name = fields.Char(string='Line No', required=True, copy=False, default=lambda self: self.env['ir.sequence'].next_by_code('pos.order.line'))