When a person who has gone through a dramatic event and keeps on reliving them, AND yet not being concious about it. With the proper psychological treatment, the person can go through those memories again without reliving the actual events over and over for the rest of their lives.
For the sake of this example, we'll say that the patient is called Mark.
Mark is a 39 year old lawyer, who has been with his firm for 10 years. Mark was witness to a shotting from a robbery gone bad. Mark in the process of trying to help the person who got shot, he also got shot on the right leg.
Mark was taken to the hospital and discharged a few days later. He went back to work next week and never took some real time off.
Mark keeps wondering why every time he hears chimes, a sirens or a similar sound, he get edgy and sometimes scared. After the urging from his boss to go seek help from a professional, he finds out that he has Post-dramatic Stress Disorder (PDSD).
The reason why Mark reacts the way he does when he hears certain sounds, is because his brain associates those sounds with the ones that took place when he was shoot.
With proper treatment, Mark was able to move behond that, and has no problems now with sounds at all.