One of the largest earthquakes ever recorded hit on Boxing Day 2004. The resulting tsunami devastated huge swaths of the Indian Ocean coastline and left an estimated quarter of a million people dead across Indonesia, Sri Lanka, India and Thailand. Aid agencies quickly arrived to help battered and traumatised survivors.
Mental health was a massive part of the emergency response but the World Health Organization promptly did something it has never done before or since. It specifically denounced a type of psychological therapy and recommended that it shouldn't be used. The therapy was a single session treatment called "psychological debriefing," which involved working with disaster victims to encourage people to supposedly "process" the intense emotions by talking through them in stages.
It was intended to prevent later mental health problems by helping people resolve difficult emotions early on. The only trouble was that it made things worse. Studies had shown that people given post-disaster psychological debriefing were subsequently more likely to suffer mental health problems than people who had had no treatment at all.