One party comes over to you and tells you the negative things someone else said or did. You experience righteous anger. This person is certainly right, the other person is certainly wrong. You might tell your source, "I agree with you. He's an awful person." You might censure the other person, "You are an awful person." You might tell others, "This person is an awful person."
Did you hear both sides in the presence of each other before forming any opinion whatsoever? If not, the Torah considers what you heard as a "false report." (The Chofetz Chaim elaborates on this in his classic work. See Prohibitions, no.2)
"The map is not the territory." This basic principle of General Semantics applies to every story and report you ever hear. Details are always left out. Words describing any situation are never an exact portrait of any interaction. What was the entire context? What were the exact words that were used by both sides? What was the exact tone of voice of each segment of the interchange? Even when someone reports what he himself said with total accuracy, the tone of voice totally transforms the energy that was exchanged. What were the facial expressions? What were the intentions, motivations, and assumptions?
King Solomon compares an outsider getting involved in an argument that is not his to a passerby who pulls the ear of a dog (Proverbs 26:17). The calm dog was harmless. Pull his ear and you'll have problems. Taking sides in a quarrel that's not yours is even worse when you don't really have the entire picture. Those who impulsively take sides often don't realize how many mistakes they will be making.
Every word an outsider says to a party in a quarrel can either make the situation better or worse. Whenever someone tells you about a situation that is either already a quarrel or could easily lead to a quarrel, ask yourself, "What can I say now that will be conducive for peace?" And when the answer is, "I don't really know," keep quiet. It's a major mistake to make things worse.
(from Rabbi Zelig Pliskin's book: Harmony with Others, p.48, www.artscroll.com)