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. 

values.update({
                    'partner_id': partner_id,
                    'move_id': move.id,
                })
                if data_type == 'product':
                    key = ('product', values['partner_id'], (values['product_id'], tuple(values['tax_ids'][0][2]), values['name']), values['analytic_account_id'], values['debit'] > 0)
                elif data_type == 'tax':
                    key = ('tax', values['partner_id'], values['tax_line_id'], values['debit'] > 0)
                elif data_type == 'counter_part':
                    key = ('counter_part', values['partner_id'], values['account_id'], values['debit'] > 0)
                else:
                    return
                grouped_data.setdefault(key, [])
                if have_to_group_by:
                    if not grouped_data[key]:
                        grouped_data[key].append(values)
                    else:
                        current_value = grouped_data[key][0]