Changing the value of the party_required field
It is currently possible to change the party_required value of an account even if there are move lines using that account. That usually brings the system to a situation which cannot be solved using the client interface because tryton constraints are rechecked with any later change of the move lines.
This should be simple to solve by checking if there are move lines using that account.
However, similar situations can happen if the user changes the 'kind' field. The account can be set to 'payable', be used in an invoice, then changed to 'expense' and be used in an invoice line. And be changed to 'payable' again. Probably, any later update of the documents created when the account was of kind 'expense' will make the application crash and most probably only solvable making updates on the database.