Your boyfriend is cheating on you.

Problem: You see your friend's boyfriend walking down the street with his arm around another girl. You call her and say "He's cheating on you."

You just proved true attribution theory. Attribution theory is the concept that people consistently make sense of the world by assigning meaning and motives to others' behavior. By assuming that your friend's boyfriend was cheating, just because he had his arm around another girl, you attributed meaning to his actions.

This becomes a problem when there is an attribution error. This is where one ascribes motivations for another's behavior to a personality or character trait when it actually results from a situational influence, or vice versa; and without a factual base. In this case, your friend's boyfriend was helping a girl who had sprained her ankle by holding her up and walking her to get help. You falsely accused him of cheating, when really he was helping.