Here is a story from the life of St. Padre Pio that illustrates it best.
There was a girl who was possessed and shouted for hours. Everybody was frightened of her. The child’s mother brought her to church hoping that Padre Pio would free the child from the evil spirit. The child also shouted a lot in the church. One day, when Padre Pio had finished hearing confessions, he met the girl that howled fearfully in front of him. The child was being held back with difficulty by two or three men. The Saint was annoyed by the whole uproar and finally said: "Stop! Enough!"
The child fell to the ground as if she was sleeping. Padre Pio told a doctor who was standing there, to bring the child to St. Michael, in the sanctuary of Monte Sant’Angelo. When the group reached the destination, they entered the cave where Saint Michael had appeared. The girl revived but nobody succeeded in bringing her near to the altar of the Angel. In the midst of the confusion, a monk took the hand of the child and touched the altar. She fell down as if she had been struck by lightning. A few minutes later she woke up and as if nothing had happened she asked her mother: "Could you buy me an ice-cream?"
At that point the group returned to St. Giovanni Rotondo in order to inform and to thank Padre Pio. But Padre Pio told her mother: "Say to your husband not to curse anymore, otherwise the demon will return."
Some would say, what does cursing hurt? It can’t be that bad. Yet, it has repercussions that caused a demon to possess this child. Even little things that we do, affect those around us, especially those entrusted to our care.