What do you put on a job application if you were fired from your last job?

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My husband had a good job and he was good at it. Unfortunately, he forget to do one important step in his job one day. It was something that may have never come to light, but could have also resulted in people getting hurt if it was not fixed. My husband realized his mistake the next day and immediately called his boss to inform him of his mistake and to ensure that the customer had the job redone ASAP. My husband could have not said anything and have just hoped that nothing bad ever happened or he could have gone to the customer behind the company's back to repair it. Instead, he was honest and stepped up and not only admitted, but informed them of the mistake. In return, they fired him. So now, he is looking for work and when he fills out applications, they all ask why he left his last job. If he says he was fired, he is likely to not even get an interview. So what should he write?