> Where did the TSA go wrong here?

Perhaps it was the bit where they held him for 18 hours without charge, due process, food, or water.

Edit: Misread the article. He was held for 3½ hours. Still, refusing to give him a glass of water was not necessary, unless water-activated throat-bombs are now a thing.

In a more general sense, TSA went wrong the same way they usually go wrong. Kafka-esque "you're not detained but you can't leave" nonsense designed to side-step due process, comically ignorant staff who think Islam is the only religion brown people can have, whilst not even being effective at detecting contraband.