CREATE TABLE `ticket` (
  `id` int(8) NOT NULL auto_increment, 
  `last_updated` datetime default NULL,
... 
)

CREATE TABLE `action` (
  `action_id` int(8) NOT NULL auto_increment,
  `ticket_id` int(8) NOT NULL default '0',
  `t_date` datetime default NULL, 
...
}

Query:
UPDATE `ticket`  SET `last_updated` = ( SELECT MAX( `t_date` ) FROM `action`  WHERE `ticket.id` = `ticket_id`)

Result:  
Unknown column 'ticket.id' in 'where clause'