Выбрать не того человека, с которым нужно иметь дело или не верный план действий; ошибиться целью.
* /If he thinks he can fool me, he is barking up the wrong tree./* /He is barking up the wrong tree when he blames his troubles on bad luck./* /The police were looking for a tall thin man, but were barking up the wrong tree; the thief was short and fat./