This cognitive distortion is when you assume that the value of a person can be equated with a single action; reacting to the label rather than the event.
Personal labelling is an extreme form of over-generalization. It means that you create a completely negative self-image based on your errors. The attitude behind it is "The measure of a man is the mistakes he makes." When you describe your mistakes using sentences that begin with "I'm a..." there is a good chance that you are personal labelling. For example, when you lose a game of tennis, you might say, "I'm a born loser" instead of, "I messed up on that game." Likewise, when you don't get the grade you expected in an exam, you might think, "I'm a failure" instead of, "I made a mistake."
Your self cannot be equated with any one thing you do; your life is a complex, ever-changing flow of thoughts, emotions, and actions. Would you ever describe yourself as an "eater" just because you eat, or a "breather" because you breathe? This is nonsense, so not only is personal labelling self-defeating, it is irrational too.
When you label other people, you will inevitably create hostility. If you label someone in your class as the "uncooperative idiot", you will resent them and jump at every chance to criticize them. In turn, they may label you, and complain at every opportunity, fuelling this vicious cycle.
Mislabelling involves describing an event with language that is inaccurate and emotionally heavily loaded. For example, a girl on a diet ate a dish of ice-cream. She thought to herself, "How repulsive of me, I'm a fat pig." This made her feel so bad, that she went on to eat a lot more of the ice-cream.
Automatic thought: I'm such a failure
Possible answers: I am not a failure, I have done many things right, but I have made one mistake. I can learn from this mistake so that I won't make it again.
Ideas taken from David D. Burns' book "Feeling Good, The New Mood Therapy"