You should not enter someone's personal space uninvited.
John is tossing a ball around with his brother and accidentally throws it into his neighbor's backyard.
John wants to get back the ball.
John knocks at his neighbor's door and asks if he can retrieve his ball.
John climbs over a fence to get into his neighbor's backyard to retrieve the ball.
His neighbor is happy to help and give him back the ball right away.
His neighbor blames him for trespassing.