Any writer’s nightmare is a copy-and-paste error that goes into print.

But it’s the rare typo that makes international news.

That’s what happened to Kellen Dwyer, an assistant U.S. attorney in the Eastern District of Virginia who inadvertently revealed WikiLeaks founder Julian Assange had been charged under seal. Dwyer twice put Assange’s name, along with a vague description of the case against him, in a document filed in what officials said was a completely unrelated child solicitation case.

The child sex case was apparently unsealed late last week; the mistake went unnoticed by lawyers, clerks and the judge who approved the order but was spotted by a national security researcher who routinely combs through federal court filings. It made public a case that prosecutors had sought to keep secret, one whose existence was tightly held both in the Alexandria office and the broader Justice Department.

At the Justice Department, officials said they were shocked by the mistake. To be sure, filing errors happen — Dwyer has in the past made small typos he has corrected in court — but not typically involving cases of the magnitude of Assange’s, the officials said.

Attorneys who’ve worked with Dwyer both at the Justice Department and in private practice described him as a professional prosecutor they never expected to be known for such a slip-up, one people who’ve spoken with him say he feels awful about.

“Kellen has the utmost integrity, I would trust him with my life. . . . This mistake is highly uncharacteristic,” said Gene Rossi, a former federal prosecutor who trained Dwyer in the Eastern District. In three decades there, he said both he and those he supervised “made quite a few mistakes in our pleadings and papers. An innocent error can surely fall through the cracks.”

The cyber unit in the Eastern District of Virginia is known for aggressively prosecuting hackers and fraud schemes even if the suspects, like Assange, can be linked to Northern Virginia only by its large population of data centers. Earlier this year, Dwyer won at trial against a Latvian computer programmer who helped hackers test their malicious software against anti-virus programs, a case that exposed connections between cybercriminals and the Russian government. The programmer’s Russian co-conspirator pleaded guilty and testified at the trial, as did a Virginia college student who used their program in creating his own program that spies on computer users’ keystrokes. Another co-conspirator, who initially argued he wasn’t responsible for the crimes people committed using his technology, pleaded guilty as well.

In those cases, Dwyer argued aggressively for people who make software used maliciously to be held responsible for all the financial losses linked to their work. Another Russian-born hacker he prosecuted went to prison for nine years after stealing thousands of credit cards and earning millions in bitcoin.

Dwyer also put in prison a woman who made over $400,000 buying designer handbags, returning fakes she bought in China and then selling the real goods online, along with a smuggler of fake Apple and Samsung products. He’s been involved in an ongoing case involving a Canadian company that sold misbranded and improperly stored Botox in the United States.

“I’ve had numerous serious and complex cases with Kellen over several years, and while a tough advocate against my clients, he has always been a consummate professional,” said Peter Greenspun, a defense attorney who often practices in Alexandria federal court. “This stuff happens all the time, particularly in the digital world.”

A spokesman for the U.S. attorney’s office in the Eastern District declined to comment, and individual prosecutors in the office are not permitted to be interviewed by the media.

Dwyer’s misstep came in a motion to keep secret a case against a man named Seitu Sulayman Kokayi, who is accused of soliciting a 15-year-old girl online. While that case involves some classified information because Kokayi was investigated for an interest in terrorism, according to court filings, officials have said there is no link to Assange.

The cyber unit in the Eastern District of Virginia prosecutes huge numbers of men for such enticement as well as for looking at, creating or attempting to create child pornography. Dwyer has worked on over a dozen cases, including that of a lawyer at a major D.C. firm, a former officer in the Coast Guard, and a retired Department of Homeland Security agent.

Identity theft and bank fraud cases are also legion, and in 2015 Dwyer prosecuted a ring of schemers led by an aspiring dental surgeon that netted over $700,000 through stolen checks and other frauds.

As an attorney with the firm Kirkland & Ellis before joining the Justice Department as a cyberprosecutor in 2014, he helped defend Facebook against shareholders who said they were kept in the dark about threats to the company’s prospects before it went public in 2012. Former co-workers said he did a substantial amount of pro bono work there, including a winning appeal to the Sixth Circuit on behalf of an convicted murderer whose confession came after being held for more than 48 hours without probable cause.

“A lot of people, me included, were not believing he could pull that off,” said Dominic Draye, a Kirkland & Ellis colleague who now serves as solicitor general for the state of Arizona. “It’s just the unhappiest thing — of all the cases in which to have a rare mistake, you couldn’t pick a worse one.”

Dwyer is also a fellow in the Leonine Forum, a conservative-leaning group of elite Catholic young professionals that meets for monthly religious discussions. After Yale Law School, he clerked for judges on both the U.S. Court of Appeals for the 9th Circuit and in the Southern District of New York.

“He worked hard, he did really solid research and writing, he was highly efficient, and just a real pleasure to work with,” said U.S. District Judge Ken Karas. “I don’t remember any mistakes.”

But, he said, such errors are common. In fact, just before speaking with this reporter, he said a lawyer had called him to say she inadvertently exposed a victim’s name in a filing.

A similar copy-and-paste error by a lawyer in his courtroom, he said, was once caught by a man with no legal training who was representing himself.

“We’re all human,” the judge said. “I make mistakes all the time.”

Matt Zapotosky contributed to this report.