I was 16 and teaching swimming and boys sports in my hometown's summer activities program. One day one of my favorite swimming students was upset and not really participating in the class like he usually did. After class I asked him what was wrong and he confided in me that his mom had caught him taking money from her purse that morning and he was still smarting from the punishment.
The next day I thoughtlessly needled him about it, including using the word thief. In front of the class. I knew that I had fucked up when he got pissed at me and stormed off, but not how badly.
The next day his mom came up to me before the class began and told me that he wouldn't be back. She asked me if I realized that her son looked up to me and thought that I hung the moon until I said that, and that it hurt him so badly that he wanted to quit swimming lessons entirely. She then told me that he was in the car, if I wanted to apologize to him. I have rarely felt so low, nor given a more sincere and heartfelt apology. He didn't accept it, and I don't blame him.
He did swap over to another swimming class and finished the summer program, so I'm happy for that. But I don't think that he ever forgave me. I know that I haven't.