In a stately, old London courtroom, Soma Sengupta was finally living her dream.

She had been admitted to a selective British law firm’s training program, wowing them with her credentials: honors at Georgetown Law School; former prosecutor for the Manhattan district attorney; defense lawyer for the Legal Aid Society. There were glowing reference letters.

She had been called to the bar in a formal ceremony at Middle Temple Hall, a grand room where little has changed since 1602, when the first known performance of Shakespeare’s “Twelfth Night” was staged there. With her dark hair covered by a barrister’s foppish white wig, she signed the registry of barristers who had stood in the same spot across the centuries, including John Rutledge, a signer of the United States Constitution, and Sir William Blackstone, the 18th-century legal scholar who wrote the seminal legal tome, “Commentaries on the Laws of England.”

She began to handle criminal cases, nearly one a day over three months. In 90 more days, she would become a full-fledged barrister, the apex of a carefully curated career.

And then it all collapsed, undone by the chance discovery of a simple lie, that led to many more. A clerk for the British firm that had accepted Ms. Sengupta stumbled upon her application file, and noticed that she had listed a date of birth that put her age at 29.