Sometimes we get annoyed at someone but we don't tell them. (Mistake #1.)  The person let you down or isn't living up to your expectations.

The thought lingers, festers. It ruminates, bubbling to the surface til we finally find someone to share our annoyance with...

My advice to you, at that moment as you're just about to speak your mind:

Bite your tongue.

(Once something is said, it can not be unsaid)

Resist the urge to gossip and talk bad about someone. Regardless of how "justified" you might feel. (Mistake #2)

Nothing good will come of this.

Best case, you get it off your chest and feel a little better. Worse case, it gets back to the person you were talking about and makes an unsatisfactory situation a whole lot worse.

If you can't tell the person directly,

(TELL THE PERSON DIRECTLY)

get the thought out of your mind.

Let it go.

*Mistake 1 Direct communication is best. Learn to not beat around the bush.

*Mistake 2 feeling "justified" is emotional bologna. It stems from a flawed/skewed way of looking at a situation and not from a practical objective viewpoint.

If you find yourself feeling "justified" in doing or saying something, know that your mindset has gone astray.

We do things because they make sense, because they're practical, because they're going to help the situation and not solely because we feel "justified" in doing it.