Usually, it is at a moment when we are most serious. We use the wrong word or a word with a double meaning. Or, we make a dramatic gesture which either knocks something over, or strikes someone else as very funny. We should be able to join in the joke against ourselves. Sometimes we are too conscious of the hurt to our dignity, We are angry with ourselves for blundering, and even more angry with other people for noticing it.
Nothing is more damaging to our sense of humour than an exaggerated sense of personal dignity.
The latter grows is pomposity's little. Brother so kill it now before it into something worse. If we want to be happy, we must cultivate the priceless gift of being able to laugh at ourselves, not bitterly, but with quiet, genuine amusement. We are all funny at times, and never more funny than when we are taking ourselves in grim, deadly earnest. If we have had the misfortune to hurt someone, we should either apologize frankly, or express our penitence to someone else who we know will pass it on to the appropriate quarter.