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. 

current_value['quantity'] = current_value.get('quantity', 0.0) + values.get('quantity', 0.0)
                        current_value['credit'] = current_value.get('credit', 0.0) + values.get('credit', 0.0)
                        current_value['debit'] = current_value.get('debit', 0.0) + values.get('debit', 0.0)
                else:
                    grouped_data[key].append(values)
            
            
            
            
            
            assert order.lines, _('The POS order must have lines when calling this method')
            
            cur = order.pricelist_id.currency_id
            for line in order.lines:
                amount = line.price_subtotal
                
                if line.product_id.property_account_income_id.id:
                    income_account = line.product_id.property_account_income_id.id
                elif line.product_id.categ_id.property_account_income_categ_id.id:
                    income_account = line.product_id.categ_id.property_account_income_categ_id.id
                else:
                    raise UserError(_('Please define income '
                                      'account for this product: "%s" (id:%d).')
                                    % (line.product_id.name, line.product_id.id))
                name = line.product_id.name
                if line.notice:
                    
                    name = name + ' (' + line.notice + ')'
                
                insert_data('product', {