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. 

if values.get('session_id'):
            
            session = self.env['pos.session'].browse(values['session_id'])
            values['name'] = session.config_id.sequence_id._next()
            values.setdefault('session_id', session.config_id.pricelist_id.id)
        else:
            
            values['name'] = self.env['ir.sequence'].next_by_code('pos.order')
        return super(PosOrder, self).create(values)
    @api.multi
    def action_view_invoice(self):
        return {
            'name': _('Customer Invoice'),
            'view_mode': 'form',
            'view_id': self.env.ref('account.invoice_form').id,
            'res_model': 'account.invoice',
            'context': "{'type':'out_invoice'}",
            'type': 'ir.actions.act_window',
            'res_id': self.invoice_id.id,
        }
    @api.multi
    def action_pos_order_paid(self):