Special Counsel Robert Mueller is reeling from a series of misfortune events. Not only has a vote to protect Mueller been blocked in the Senate, Paul Manafort has been working with the Trump campaign over the Mueller team this entire time.

On Monday, Special Counsel Mueller filed an incendiary status report accusing Manafort of breaching their cooperation agreement, lying to the F.B.I. and committing further federal crimes after his September guilty plea to fraud and other charges. Mueller’s team held off Manafort’s sentencing while he supposedly cooperated — and while President Trump completed his written responses to questions from Mueller’s team of prosecutors. Now, Mueller asked the federal judges hearing his cases in Washington and Virginia to sentence him promptly on what are certain to be very harsh terms. Mueller will be documenting his accusations publicly in his sentencing position papers.

On Tuesday, The Times reported that Manafort’s lawyers have been telling Trump’s lawyers what the special counsel asked Manafort and what the Manafort said in response.

Manafort violating his cooperation agreement isn’t much of a surprise. Wise prosecutors know that cooperating witnesses can be notoriously unreliable. It’s not unusual for prosecutors to catch cooperators in lies or recidivism.

Yet, the fact that Manafort’s lawyers have been spilling the details of their client’s cooperation to Trump lawyers under the cover of an often-used but little-understood pact called a joint defense agreement — is shocking. The revelation is catastrophic for Mueller.

It’s a HUGE blow to Mueller’s team, because their questions to Manafort, repeated to Trump’s lawyers, may be a road guide to part of the special counsel investigation. Trump’s lawyers can now adjust their defense, and the president’s responses, based on what they’ve learned about Mueller’s focus and what he knows or doesn’t know. With their filing on Monday asserting Manafort’s dishonesty, the prosecutors lost him as a cooperating witness and can no longer pursue any theory relying on his testimony.

It’s a small blow to Manafort, who will receive no sentencing credit for his brief cooperation. It’s a blow to Manafort lawyers; for no federal prosecutor will ever trust them again. It could only be bad for Trump because Mueller may now be able to delve into the Trump lawyers’ conversations with Manafort’s lawyers.

Manafort, like many of the hapless former luminaries snared in Mueller’s investigation, had a joint defense agreement with Trump. Such a pact lets defense lawyers exchange information without waiving attorney-client privilege.

Under normal circumstances, if a lawyer reveals what a client said in confidence, or reveals strategy and analysis of a case, that information is no longer confidential, and the government can compel testimony about it. A joint defense agreement allows lawyers for people with a common interest in a case to share what they learned from clients without that information losing its confidential nature. Because everyone in the agreement has a common interest in defending the case and has agreed to keep the information secret, the theory goes, sharing the information within the group doesn’t waive its confidentiality.

Joint defense agreements are common in white-collar investigations because they allow lawyers to figure out two crucial things: what happened and what the prosecutor knows about it. Trump’s lawyers have made vigorous use of such pacts — we’ve learned he has them with Manafort and with the Roger Stone confidant Jerome Corsi.

Yet the pacts almost always explicitly require the parties to withdraw from the agreement immediately if they cooperate with the government. After all, once someone begins to cooperate, his interests are no longer common with the other members of the joint defense agreement — by definition, they’re now adverse. This is often how we learn that someone will soon plead guilty: The former national security adviser Michael Flynn had a defense pact with the president, but his withdrawal from it signaled his cooperation and plea.

The objectives of a cooperation agreement like the one Manafort entered with Mueller are flatly inconsistent with the obligations of a joint defense agreement. So it doesn’t matter whether or not Manafort explicitly withdrew from his deal with Trump, because once he began cooperating — or, at least, pretending to — the legal theory supporting the agreement collapsed. Because Manafort no longer had, on paper, a “common interest” with Trump, his lawyers’ communications with Trump’s lawyers could no longer be seen as cloaked with any expectation of confidentiality.

If they revealed client confidences, they waived the attorney-client privilege. Mueller can potentially try to find out what Trump’s and Manafort’s lawyers said to one another about their respective clients after Manafort began cooperating. The resulting legal battles over privilege would be one for the books.

Some analysts speculate that Mueller intended this result — that he knew that Manafort would lie to him, knew that Manafort’s lawyers would brief Trump on those lies, and knew that Trump would repeat those lies in his written statement to Mueller, thus committing a new federal crime. This, however, seems very unlikely.

That’s a good plot for a legal action thriller, but it’s not how real federal prosecutors work. Mueller is a run-of-the-mill sort of prosecutor, and not one to think up such complex ploys. To the extent he trusted Manafort and revealed details of his investigation, he made a mistake. Of course, the president’s team — never a font of shrewd criminal defense strategy — may have made a mistake, too, if they incorporated Manafort’s lies into their own written responses to Mueller’s questions. That could expand the scope of Mueller’s investigation to include new false statements to the F.B.I., the downfall of several of his targets.

Mueller’s mistake is understandable, yet amateurish. Manafort’s lawyers’ communications with Trump’s lawyers are shocking and unprecedented, a brazen violation of criminal defense norms notable even in an investigation full of them. They are consistent with only one conclusion: Manafort and his lawyers seek a presidential pardon, not a reduced sentence through sincere cooperation.