The demise of the perjury trial of Roger Clemens was sown in one of the most routine moments of any prosecution: the playing of a video for jurors.

It actually was the fifth cued up that day, showing Clemens’s testimony before a House committee. While tedious, the clips were an essential part of the Justice Department’s case that the baseball legend had lied to Congress in 2008, when he told lawmakers that he had never taken performance-enhancing drugs.

Within moments of the tape’s rolling, the trial was over, and the prosecutors sat slumped and dejected in their chairs.

A judge had declared a mistrial, ruling that the tape included evidence he had barred from the jury.

The dramatic decision left legal observers wondering how such a high-profile prosecution could end so abruptly, on just the second day of testimony. A review of transcripts and interviews with people knowledgeable about what happened reveals that federal prosecutors did not intentionally introduce barred evidence to the jury. Despite years of experience, two well-respected prosecutors had made a basic mistake and not carefully reviewed the videos they planned to show jurors.

Their error was intensified because it occurred in front of a by-the-book judge who noticed it before defense lawyers could even raise an objection.

“Because of the prosecutors’ excellent reputations, you have to believe it’s a mistake, just a mistake,” said Tom Zeno, a former colleague of the two government lawyers. “What compounds the error here,” he added, “is that the judge runs a very tight ship and had high expectations of the prosecutors.”

The judge in the case is Reggie B. Walton, who will be weighing legal arguments in coming weeks to determine whether to grant prosecutors a retrial. Last week, the judge gave no hint about how he might rule.

First appointed to the D.C. Superior Court by President Ronald Reagan in 1981, Walton was tapped for the federal bench in 2001 by President George W. Bush. Since then, he has earned a reputation for being able to manage complex and high-profile trials.

In 2007, he oversaw the trial of I. Lewis “Scooter” Libby, who was convicted of lying about his role in the leak of an undercover CIA officer's identity. He sentenced Libby to 2 ½ years in prison — a sentence later commuted by Bush.

A former fullback on his college football team, he also isn’t afraid to take action. In 2005, he broke up an assault in Chevy Chase Circle by tackling the assailant.

“He’s a very solid, no-nonsense judge,” said Chief U.S. District Court Judge Royce C. Lamberth. “He did an outstanding job in the trial of Mr. Libby.”

Citing a judicial gag order, the two prosecutors in the Clemens case — Steven Durham and Daniel Butler — declined to comment. Former colleagues and defense lawyers said the men are straight shooters.

“Steve is a boy scout,” said Preston Burton, a former federal prosecutor now in private practice. “He is principled, fair and decent. Dan is an honest guy. Both are hard-working prosecutors with a lot of integrity.”

Durham, a graduate of Northwestern University School of Law, joined the District’s U.S. Attorney’s Office in 1992 and has been in charge of its public corruption prosecutions since 2007.

Butler, who joined the Justice Department in 1981, has been specializing in fraud and public corruption for the past seven years and led the successful prosecution of Deborah Jeane Palfrey, known as the D.C. Madam, in 2008. Butler, who lost the use of his legs when he was struck by a car in the 1970s, is also an accomplished athlete, winning three gold medals in swimming at the 1996 Paralympic Games in Atlanta.

Durham and Butler began investigating Clemens after the Justice Department received a referral from Congress seeking an investigation into the star pitcher’s testimony in 2008 to the House Committee on Oversight and Government Reform. Just months after a report by former senator George Mitchell linked Clemens and dozens of other players to the use of performance-enhancing drugs, the pitcher testified before the House panel. In dramatic fashion, Clemens denied ever having taken steroids or human growth hormone, known as HGH for short.

At the same hearing, the pitcher’s chief accuser — his former trainer Brian McNamee — testified that he had injected the “Rocket” with those substances with the pitcher’s knowledge.

After lengthy grand jury investigation, Clemens was indicted on six counts of perjury, obstruction of Congress and making false statements.

As Thursday started in the District’s federal court, prosecutors began playing video clips of the hearing before the House oversight committee. The tapes showed Clemens taking the oath, denying the use of steroids and discussing how McNamee had given him shots of vitamin B-12 and a painkiller — not steroids or HGH.

The fifth video clip featured Clemens telling Rep. Elijah E. Cummings (D-Md.) that a friend and former teammate, Andy Pettitte, had misheard or misremembered a conversation they had in 1999 or 2000. Pettitte had told Congressional investigators that Clemens had confided in him that he had taken HGH. The indictment alleges that Clemens obstructed Congress with those statements.

On the tape, Cummings also discussed Pettitte’s disclosure and then asked Clemens about the recollection of Pettitte’s wife. In an affidavit for the House panel, she disclosed that her husband had relayed his conversation with Clemens back in 1999 or 2000.

Walton suddenly stopped the proceeding, and called prosecutors and defense lawyers to the bench.

Just a week earlier, he had ruled that prosecutors could not produce any information to the jury related to Laura Pettitte and her conversation with her husband. He had determined that her version of events was hearsay.

At the bench, Walton asked Rusty Hardin, Clemens’s lead attorney, whether he was going to object to the statement. “If you’re not objecting, I just want to get it on the record that you’ve made a tactical decision,” the judge said.

Hardin replied that he didn’t “know quite what to do in front of the jury” because the entire transcript of the House hearing had already been introduced into evidence. After a moment, the defense lawyer objected to the video.

Prosecutors argued that they had turned over the videos to the defense in early May, and Hardin had not objected when they were introduced into evidence that morning. “There is no bad faith here on the part of the government in trying to prove this case,” pleaded Durham, who had been admonished during opening statements when Walton felt the prosecutor had presented prohibited information to the jury.

Clearly troubled by what had just happened in his courtroom, Walton asked Hardin what he wanted to do, and the defense lawyer said that “the best I can suggest is the court’s admonition to the jury that they are to disregard” the video. The judge left the courtroom for 11 minutes.

Upon the judge’s return, Hardin had changed his mind and requested a mistrial.

Walton agreed. Not only did prosecutors introduce barred information, the judge said, but they also had played a video in which a congressman was praising the credibility of a key prosecution witness, also a problem. The judge also didn’t understand why prosecutors didn’t redact references to Laura Pettitte from the video.

Walton said such statements were not fair to the defense because they made it more difficult for Clemens’s lawyers to attack Pettitte on the stand. He then turned his frustration on prosecutors.

“A first-year law student would know you can’t bolster one witness . . . with clearly inadmissible evidence,” the judge said before declaring the mistrial.

After the ruling, the two prosecutors sat silently at their courtroom table, staring at their hands and paperwork. According to their friends, the two crestfallen men realized that Walton was right in one respect. The prosecutors had committed a rookie blunder: They hadn’t reviewed the videos in light of the judge’s rulings before playing them in court.