When the archbishop of Canterbury, the Most Rev. Justin Welby, welcomed friends to sing Christmas carols at his London residence last week, his remarks ran, as they often do, to his coronation of King Charles III in May.

The vaulted chamber in which his guests were gathered, he told them, had been used to rehearse the ceremony twice a week over four months. Members of his staff were assigned to play Charles and other royals in a rotating cast. “I always played the archbishop,” he said dryly.

Then he ran through the script a few times with the actual king. “We practiced putting it on and screwing it down,” Archbishop Welby said later of the 17th-century St. Edward’s Crown. “It’s a wobbly old thing.”

But on coronation day, before a hushed assembly of 2,300 and a worldwide television audience of hundreds of millions, the archbishop made one conspicuous error: He bent down after placing the crown to inspect whether it was sitting level on the sovereign’s head, an unscripted move that made him look vaguely like a carpenter inspecting his work. “I got it right,” he recalled. “I just didn’t trust myself.”