Works:
SELECT COUNT(*)
  FROM
    istria_acc acc
  LEFT JOIN
    istria_acc_bill bill
  ON
    bill.accomodation=acc.id AND pay_date='0000-00-00'

Get an error that acc.id doesn't exist:
SELECT COUNT(*)
  FROM
    istria_acc acc,
    istria_city c
  LEFT JOIN
    istria_acc_bill bill
  ON
    bill.accomodation=acc.id AND pay_date='0000-00-00'